Hi
I have an equal situation.
i have a reverse Proxy on server 1 (apache -Debian Raspian) wich is directly behind my router (port 80 and 443)
on my Server 2 runs nextcloud and docker with code (apache - ubuntu 16.04).
i use letsencrypt to run everything with ssl on my Server 1
my apache conf on Server 1 is like the standard conf from collabora
i only changed the localhost ip from 127.0.0.1 to the ip of my Server 2
like this:
ProxyPass /loleaflet https://server2:9980/loleaflet retry=0
ProxyPassReverse /loleaflet https://server2:9980/loleaflet
to access the docker container i started it a little bit different then the manual to get this :
-> docker ps
0.0.0.0:9980->9980/tcp
here is my run command :
docker run -t -d -p 9980:9980 -e ‘domain=subdomain1\.domain1\.de|subdomain2\.domain2\.de’ --restart always --name code --cap-add MKNOD collabora/code
it is important to use your nextcloud domain and not your office domain !
the last step is on the admin page on nextcloud.
setting up the the right office domain (incl. the port)
like this: https://officedomain.de:443
thats my workaround
i hope this is useful
sorry for my bad english 