I tell my solution myself
Because of the issue from #7:
i edit /etc/apache2/conf-available/security.conf:
Header set X-Frame-Options: "allow-from https://nextcloud.mydomain.com/"
Now i get the “Unauthorized WOPI host”
Then
docker ps
then
docker kill CONTAINER ID
then
docker run -t -d -p 127.0.0.1:9980:9980 -e 'domain=cloud\\.nextcloud\\.com' -e 'username=admin' -e 'password=123456' --restart always --cap-add MKNOD collabora/code
Now it seems it works! System is nextcloud 10.0.2
I hope when i go to 11.0. there is no confliction.
Thanks on the authors in https://help.nextcloud.com/t/collabora-online-setup/4645/10
Especially Lars M and Neso, there answer help me!