Help needed: Frequent 500 crashes on NC docker setup

I am using a docker setup very similar to what has been recommended in the guides.

docker run -d \
    --restart=always \
    --name mariadb \
    -v /opt/dockerfiles/mariadb:/var/lib/mysql \
    -e "MYSQL_ROOT_PASSWORD=xxx" \
    -e "MYSQL_DATABASE=nextcloud" \
    -e "MYSQL_USER=nextcloud" \
    -e "MYSQL_PASSWORD=xxx" \
    mariadb

docker run -d \
    --restart=always \
    --name nextcloud \
    -e "VIRTUAL_HOST=xxx" \
    -e "LETSENCRYPT_HOST=xxx" \
    -e "LETSENCRYPT_EMAIL=xxx" \
    -e "MYSQL_DATABASE=nextcloud" \
    -e "MYSQL_USER=nextcloud" \
    -e "MYSQL_PASSWORD=xxx" \
    -e "MYSQL_HOST=172.17.0.2:3306" \
    -e "NEXTCLOUD_ADMIN_USER=xxx" \
    --expose 80 \
    --expose 443 \
    -v /opt/dockerfiles/nextcloud:/var/www/html \
    --link mariadb \
    nextcloud

and quite frequently, I get this error page:

Internal Server Error

The server encountered an internal error and was unable to complete your request.
Please contact the server administrator if this error reappears multiple times, please include the technical details below in your report.
More details can be found in the server log.

Peeking into the log file in data/nextcloud.log, I see these error when I try to connect

{"reqId":"UunwhBdkaEw5gpxWwz9U","level":3,"time":"2020-02-17T18:15:03+00:00","remoteAddr":"80.113.225.46","user":"kianenigma","app":"core","method":"GET","url":"/core/preview?fileId=1481&c=36a6e1dd4ce5c534be276406f6b6af4b&x=250&y=250&forceIcon=0","message":{"Exception":"Doctrine\\DBAL\\Exception\\DriverException","Message":"An exception occurred while executing 'SELECT `filecache`.`fileid`, `storage`, `path`, `path_hash`, `filecache`.`parent`, `name`, `mimetype`, `mimepart`, `size`, `mtime`, `storage_mtime`, `encrypted`, `etag`, `permissions`, `checksum`, `metadata_etag`, `creation_time`, `upload_time` FROM `oc_filecache` `filecache` LEFT JOIN `oc_filecache_extended` `fe` ON `filecache`.`fileid` = `fe`.`fileid` WHERE (`storage` = ?) AND (`path_hash` = ?)' with params [1, \"959d195e7116732c95a3b1c5b06cac25\"]:\n\nSQLSTATE[HY000]: General error: 2006 MySQL server has gone away","Code":0,"Trace":[{"file":"/var/www/html/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/DBALException.php","line":169,"function":"convertException","class":"Doctrine\\DBAL\\Driver\\AbstractMySQLDriver","type":"->","args":["An exception occurred while executing 'SELECT `filecache`.`fileid`, `storage`, `path`, `path_hash`, `filecache`.`parent`, `name`, `mimetype`, `mimepart`, `size`, `mtime`, `storage_mtime`, `encrypted`, `etag`, `permissions`, `checksum`, `metadata_etag`, `creation_time`, `upload_time` FROM `oc_filecache` `filecache` LEFT JOIN `oc_filecache_extended` `fe` ON `filecache`.`fileid` = `fe`.`fileid` WHERE (`storage` = ?) AND (`path_hash` = ?)' with params [1, \"959d195e7116732c95a3b1c5b06cac25\"]:\n\nSQLSTATE[HY000]: General error: 2006 MySQL server has gone away",{"errorInfo":["HY000",2006,"MySQL server has gone away"],"__class__":"Doctrine\\DBAL\\Driver\\PDOException"}]},{"file":"/var/www/html/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/DBALException.php","line":145,"function":"wrapException","class":"Doctrine\\DBAL\\DBALException","type":"::","args":[{"__class__":"Doctrine\\DBAL\\Driver\\PDOMySql\\Driver"},{"errorInfo":["HY000",2006,"MySQL server has gone away"],"__class__":"Doctrine\\DBAL\\Driver\\PDOException"},"An exception occurred while executing 'SELECT `filecache`.`fileid`, `storage`, `path`, `path_hash`, `filecache`.`parent`, `name`, `mimetype`, `mimepart`, `size`, `mtime`, `storage_mtime`, `encrypted`, `etag`, `permissions`, `checksum`, `metadata_etag`, `creation_time`, `upload_time` FROM `oc_filecache` `filecache` LEFT JOIN `oc_filecache_extended` `fe` ON `filecache`.`fileid` = `fe`.`fileid` WHERE (`storage` = ?) AND (`path_hash` = ?)' with params [1, \"959d195e7116732c95a3b1c5b06cac25\"]:\n\nSQLSTATE[HY000]: General error: 2006 MySQL server has gone away"]},{"file":"/var/www/html/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Connection.php","line":911,"function":"driverExceptionDuringQuery","class":"Doctrine\\DBAL\\DBALException","type":"::","args":[{"__class__":"Doctrine\\DBAL\\Driver\\PDOMySql\\Driver"},{"errorInfo":["HY000",2006,"MySQL server has gone away"],"__class__":"Doctrine\\DBAL\\Driver\\PDOException"},"SELECT `filecache`.`fileid`, `storage`, `path`, `path_hash`, `filecache`.`parent`, `name`, `mimetype`, `mimepart`, `size`, `mtime`, `storage_mtime`, `encrypted`, `etag`, `permissions`, `checksum`, `metadata_etag`, `creation_time`, `upload_time` FROM `oc_filecache` `filecache` LEFT JOIN `oc_filecache_extended` `fe` ON `filecache`.`fileid` = `fe`.`fileid` WHERE (`storage` = ?) AND (`path_hash` = ?)",{"1":1,"2":"959d195e7116732c95a3b1c5b06cac25"}]},{"file":"/var/www/html/lib/private/DB/Connection.php","line":199,"function":"executeQuery","class":"Doctrine\\DBAL\\Connection","type":"->","args":["SELECT `filecache`.`fileid`, `storage`, `path`, `path_hash`, `filecache`.`parent`, `name`, `mimetype`, `mimepart`, `size`, `mtime`, `storage_mtime`, `encrypted`, `etag`, `permissions`, `checksum`, `metadata_etag`, `creation_time`, `upload_time` FROM `oc_filecache` `filecache` LEFT JOIN `oc_filecache_extended` `fe` ON `filecache`.`fileid` = `fe`.`fileid` WHERE (`storage` = ?) AND (`path_hash` = ?)",[1,"959d195e7116732c95a3b1c5b06cac25"],[1,2],null]},{"file":"/var/www/html/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Query/QueryBuilder.php","line":200,"function":"executeQuery","class":"OC\\DB\\Connection","type":"->","args":["SELECT `filecache`.`fileid`, `storage`, `path`, `path_hash`, `filecache`.`parent`, `name`, `mimetype`, `mimepart`, `size`, `mtime`, `storage_mtime`, `encrypted`, `etag`, `permissions`, `checksum`, `metadata_etag`, `creation_time`, `upload_time` FROM `oc_filecache` `filecache` LEFT JOIN `oc_filecache_extended` `fe` ON `filecache`.`fileid` = `fe`.`fileid` WHERE (`storage` = :dcValue1) AND (`path_hash` = :dcValue2)",{"dcValue1":1,"dcValue2":"959d195e7116732c95a3b1c5b06cac25"},{"dcValue1":1,"dcValue2":2}]},{"file":"/var/www/html/lib/private/DB/QueryBuilder/QueryBuilder.php","line":215,"function":"execute","class":"Doctrine\\DBAL\\Query\\QueryBuilder","type":"->","args":[]},{"file":"/var/www/html/lib/private/Files/Cache/Cache.php","line":153,"function":"execute","class":"OC\\DB\\QueryBuilder\\QueryBuilder","type":"->","args":[]},{"file":"/var/www/html/lib/private/Files/View.php","line":1329,"function":"get","class":"OC\\Files\\Cache\\Cache","type":"->","args":["appdata_oc0k492rwdti/js/core"]},{"file":"/var/www/html/lib/private/Files/View.php","line":1384,"function":"getCacheEntry","class":"OC\\Files\\View","type":"->","args":[{"cache":{"__class__":"OC\\Files\\Cache\\Cache"},"scanner":{"__class__":"OC\\Files\\Cache\\Scanner"},"watcher":null,"propagator":{"__class__":"OC\\Files\\Cache\\Propagator"},"updater":{"__class__":"OC\\Files\\Cache\\Updater"},"__class__":"OCA\\Files_Trashbin\\Storage"},"appdata_oc0k492rwdti/js/core","/appdata_oc0k492rwdti/js/core"]},{"file":"/var/www/html/lib/private/Files/Node/Root.php","line":199,"function":"getFileInfo","class":"OC\\Files\\View","type":"->","args":["/appdata_oc0k492rwdti/js/core"]},{"function":"get","class":"OC\\Files\\Node\\Root","type":"->","args":["/appdata_oc0k492rwdti/js/core"]},{"file":"/var/www/html/lib/private/Files/Node/LazyRoot.php","line":65,"function":"call_user_func_array","args":[[{"__class__":"OC\\Files\\Node\\Root"},"get"],["appdata_oc0k492rwdti/js/core"]]},{"file":"/var/www/html/lib/private/Files/Node/LazyRoot.php","line":142,"function":"__call","class":"OC\\Files\\Node\\LazyRoot","type":"->","args":["get",["appdata_oc0k492rwdti/js/core"]]},{"file":"/var/www/html/lib/private/Files/AppData/AppData.php","line":143,"function":"get","class":"OC\\Files\\Node\\LazyRoot","type":"->","args":["appdata_oc0k492rwdti/js/core"]},{"file":"/var/www/html/lib/private/Template/JSCombiner.php","line":96,"function":"getFolder","class":"OC\\Files\\AppData\\AppData","type":"->","args":["core"]},{"file":"/var/www/html/lib/private/Template/JSResourceLocator.php","line":115,"function":"process","class":"OC\\Template\\JSCombiner","type":"->","args":["/var/www/html","core/js/merged-template-prepend.json","core"]},{"file":"/var/www/html/lib/private/Template/JSResourceLocator.php","line":71,"function":"cacheAndAppendCombineJsonIfExist","class":"OC\\Template\\JSResourceLocator","type":"->","args":["/var/www/html","core/js/merged-template-prepend.json"]},{"file":"/var/www/html/lib/private/Template/ResourceLocator.php","line":78,"function":"doFind","class":"OC\\Template\\JSResourceLocator","type":"->","args":["js/merged-template-prepend"]},{"file":"/var/www/html/lib/private/TemplateLayout.php","line":346,"function":"find","class":"OC\\Template\\ResourceLocator","type":"->","args":[["core/js/dist/main","js/merged-template-prepend","search/js/search","core/l10n/en","files_pdfviewer/l10n/en","files_pdfviewer/js/previewplugin","files_sharing/l10n/en","files_sharing/js/dist/main","files_videoplayer/l10n/en","files_videoplayer/js/main","notifications/l10n/en","notifications/js/notifications","search/l10n/en","search/js/searchprovider","js/files/fileinfo","js/files/client"]]},{"file":"/var/www/html/lib/private/TemplateLayout.php","line":174,"function":"findJavascriptFiles","class":"OC\\TemplateLayout","type":"::","args":[["core/js/dist/main","js/merged-template-prepend","search/js/search","core/l10n/en","files_pdfviewer/l10n/en","files_pdfviewer/js/previewplugin","files_sharing/l10n/en","files_sharing/js/dist/main","files_videoplayer/l10n/en","files_videoplayer/js/main","notifications/l10n/en","notifications/js/notifications","search/l10n/en","search/js/searchprovider","js/files/fileinfo","js/files/client"]]},{"file":"/var/www/html/lib/private/legacy/template.php","line":184,"function":"__construct","class":"OC\\TemplateLayout","type":"->","args":["error",""]},{"file":"/var/www/html/lib/private/Template/Base.php","line":132,"function":"fetchPage","class":"OC_Template","type":"->","args":[]},{"file":"/var/www/html/lib/private/legacy/template.php","line":333,"function":"printPage","class":"OC\\Template\\Base","type":"->","args":[]},{"file":"/var/www/html/index.php","line":65,"function":"printExceptionErrorPage","class":"OC_Template","type":"::","args":[{"__class__":"Doctrine\\DBAL\\Exception\\DriverException"},500]}],"File":"/var/www/html/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Driver/AbstractMySQLDriver.php","Line":106,"Previous":{"Exception":"Doctrine\\DBAL\\Driver\\PDOException","Message":"SQLSTATE[HY000]: General error: 2006 MySQL server has gone away","Code":"HY000","Trace":[{"file":"/var/www/html/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Connection.php","line":903,"function":"execute","class":"Doctrine\\DBAL\\Driver\\PDOStatement","type":"->","args":[]},{"file":"/var/www/html/lib/private/DB/Connection.php","line":199,"function":"executeQuery","class":"Doctrine\\DBAL\\Connection","type":"->","args":["SELECT `filecache`.`fileid`, `storage`, `path`, `path_hash`, `filecache`.`parent`, `name`, `mimetype`, `mimepart`, `size`, `mtime`, `storage_mtime`, `encrypted`, `etag`, `permissions`, `checksum`, `metadata_etag`, `creation_time`, `upload_time` FROM `oc_filecache` `filecache` LEFT JOIN `oc_filecache_extended` `fe` ON `filecache`.`fileid` = `fe`.`fileid` WHERE (`storage` = ?) 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General error: 2006 MySQL server has gone away","Code":"HY000","Trace":[{"file":"/var/www/html/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Driver/PDOStatement.php","line":117,"function":"execute","class":"PDOStatement","type":"->","args":[null]},{"file":"/var/www/html/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Connection.php","line":903,"function":"execute","class":"Doctrine\\DBAL\\Driver\\PDOStatement","type":"->","args":[]},{"file":"/var/www/html/lib/private/DB/Connection.php","line":199,"function":"executeQuery","class":"Doctrine\\DBAL\\Connection","type":"->","args":["SELECT `filecache`.`fileid`, `storage`, `path`, `path_hash`, `filecache`.`parent`, `name`, `mimetype`, `mimepart`, `size`, `mtime`, `storage_mtime`, `encrypted`, `etag`, `permissions`, `checksum`, `metadata_etag`, `creation_time`, `upload_time` FROM `oc_filecache` `filecache` LEFT JOIN `oc_filecache_extended` `fe` ON `filecache`.`fileid` = `fe`.`fileid` WHERE (`storage` = ?) AND (`path_hash` = ?)",[1,"959d195e7116732c95a3b1c5b06cac25"],[1,2],null]},{"file":"/var/www/html/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Query/QueryBuilder.php","line":200,"function":"executeQuery","class":"OC\\DB\\Connection","type":"->","args":["SELECT `filecache`.`fileid`, `storage`, `path`, `path_hash`, `filecache`.`parent`, `name`, `mimetype`, `mimepart`, `size`, `mtime`, `storage_mtime`, `encrypted`, `etag`, `permissions`, `checksum`, `metadata_etag`, `creation_time`, `upload_time` FROM `oc_filecache` `filecache` LEFT JOIN `oc_filecache_extended` `fe` ON `filecache`.`fileid` = `fe`.`fileid` WHERE (`storage` = :dcValue1) AND (`path_hash` = :dcValue2)",{"dcValue1":1,"dcValue2":"959d195e7116732c95a3b1c5b06cac25"},{"dcValue1":1,"dcValue2":2}]},{"file":"/var/www/html/lib/private/DB/QueryBuilder/QueryBuilder.php","line":215,"function":"execute","class":"Doctrine\\DBAL\\Query\\QueryBuilder","type":"->","args":[]},{"file":"/var/www/html/lib/private/Files/Cache/Cache.php","line":153,"function":"execute","class":"OC\\DB\\QueryBuilder\\QueryBuilder","type":"->","args":[]},{"file":"/var/www/html/lib/private/Files/View.php","line":1329,"function":"get","class":"OC\\Files\\Cache\\Cache","type":"->","args":["appdata_oc0k492rwdti/js/core"]},{"file":"/var/www/html/lib/private/Files/View.php","line":1384,"function":"getCacheEntry","class":"OC\\Files\\View","type":"->","args":[{"cache":{"__class__":"OC\\Files\\Cache\\Cache"},"scanner":{"__class__":"OC\\Files\\Cache\\Scanner"},"watcher":null,"propagator":{"__class__":"OC\\Files\\Cache\\Propagator"},"updater":{"__class__":"OC\\Files\\Cache\\Updater"},"__class__":"OCA\\Files_Trashbin\\Storage"},"appdata_oc0k492rwdti/js/core","/appdata_oc0k492rwdti/js/core"]},{"file":"/var/www/html/lib/private/Files/Node/Root.php","line":199,"function":"getFileInfo","class":"OC\\Files\\View","type":"->","args":["/appdata_oc0k492rwdti/js/core"]},{"function":"get","class":"OC\\Files\\Node\\Root","type":"->","args":["/appdata_oc0k492rwdti/js/core"]},{"file":"/var/www/html/lib/private/Files/Node/LazyRoot.php","line":65,"function":"call_user_func_array","args":[[{"__class__":"OC\\Files\\Node\\Root"},"get"],["appdata_oc0k492rwdti/js/core"]]},{"file":"/var/www/html/lib/private/Files/Node/LazyRoot.php","line":142,"function":"__call","class":"OC\\Files\\Node\\LazyRoot","type":"->","args":["get",["appdata_oc0k492rwdti/js/core"]]},{"file":"/var/www/html/lib/private/Files/AppData/AppData.php","line":143,"function":"get","class":"OC\\Files\\Node\\LazyRoot","type":"->","args":["appdata_oc0k492rwdti/js/core"]},{"file":"/var/www/html/lib/private/Template/JSCombiner.php","line":96,"function":"getFolder","class":"OC\\Files\\AppData\\AppData","type":"->","args":["core"]},{"file":"/var/www/html/lib/private/Template/JSResourceLocator.php","line":115,"function":"process","class":"OC\\Template\\JSCombiner","type":"->","args":["/var/www/html","core/js/merged-template-prepend.json","core"]},{"file":"/var/www/html/lib/private/Template/JSResourceLocator.php","line":71,"function":"cacheAndAppendCombineJsonIfExist","class":"OC\\Template\\JSResourceLocator","type":"->","args":["/var/www/html","core/js/merged-template-prepend.json"]},{"file":"/var/www/html/lib/private/Template/ResourceLocator.php","line":78,"function":"doFind","class":"OC\\Template\\JSResourceLocator","type":"->","args":["js/merged-template-prepend"]},{"file":"/var/www/html/lib/private/TemplateLayout.php","line":346,"function":"find","class":"OC\\Template\\ResourceLocator","type":"->","args":[["core/js/dist/main","js/merged-template-prepend","search/js/search","core/l10n/en","files_pdfviewer/l10n/en","files_pdfviewer/js/previewplugin","files_sharing/l10n/en","files_sharing/js/dist/main","files_videoplayer/l10n/en","files_videoplayer/js/main","notifications/l10n/en","notifications/js/notifications","search/l10n/en","search/js/searchprovider","js/files/fileinfo","js/files/client"]]},{"file":"/var/www/html/lib/private/TemplateLayout.php","line":174,"function":"findJavascriptFiles","class":"OC\\TemplateLayout","type":"::","args":[["core/js/dist/main","js/merged-template-prepend","search/js/search","core/l10n/en","files_pdfviewer/l10n/en","files_pdfviewer/js/previewplugin","files_sharing/l10n/en","files_sharing/js/dist/main","files_videoplayer/l10n/en","files_videoplayer/js/main","notifications/l10n/en","notifications/js/notifications","search/l10n/en","search/js/searchprovider","js/files/fileinfo","js/files/client"]]},{"file":"/var/www/html/lib/private/legacy/template.php","line":184,"function":"__construct","class":"OC\\TemplateLayout","type":"->","args":["error",""]},{"file":"/var/www/html/lib/private/Template/Base.php","line":132,"function":"fetchPage","class":"OC_Template","type":"->","args":[]},{"file":"/var/www/html/lib/private/legacy/template.php","line":333,"function":"printPage","class":"OC\\Template\\Base","type":"->","args":[]},{"file":"/var/www/html/index.php","line":65,"function":"printExceptionErrorPage","class":"OC_Template","type":"::","args":[{"__class__":"Doctrine\\DBAL\\Exception\\DriverException"},500]}],"File":"/var/www/html/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Driver/PDOStatement.php","Line":117}},"CustomMessage":"--"},"userAgent":"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.87 Safari/537.36","version":"18.0.0.10"}

Which suggests something is wrong in the database

Message":"SQLSTATE[HY000]: General error: 2006 MySQL server has gone away","Code":"HY000"

but I can’t quite pursue it further. docker logs mariadb doesn’t show anything that looks bad.

Any help is highly appreciated!
Thanks!

The reason was MySQL was running out of memory. Resized the droplet and works fine now.