I have been working on and testing the OTRS 6 update to my unofficial OTRS docker images for the past month, and everything seems to be working fine with the new OTRS version.
The first thing done was to update the container base image to CentOS 7. I had avoided this update in the past because I have had some issues with the apache server from CentOS 7. I remember there was a bug that prevented the httpd process from starting, and also some of the reconfiguration that the container startup script does to the apache server needed to be rewritten, so I opted to wait until version 6 was released (as I expected to be the minimum supported CentOS version).
Now external databases can be used by setting the following environment variables:
The docker-compose file format was updated to version 3 (about time), but I left the old v1 on the repo if needed. Also there's a new CHANGELOG file, tracking most significant changes back to the first OTRS 5 image (check it out for a more detailed feature/bugfixing list).
From now it's not possible anymore to set some configuration options using environment variables, like OTRS_POSTMASTER_FETCH_TIME because it's not possible anymore to set the postmaster's fetch time from command line as it was possible until OTRS 5. Others variables were removed because after being set on Config.pm vía env variables, they could not be changed later using SysConfig, so the ones that could be edited at a later time were removed, like OTRS_ADMIN_EMAIL, OTRS_ORGANIZATION and OTRS_SYSTEM_ID (actually, these last ones were removed at some point during 5.0.x images, check the CHANGELOG for the exact version).
Another feature added during 5.0.x development was the auto module reinstallation at container bootup after version upgrade. So, if you had some additional modules installed and you upgraded your OTRS image to the latest version, they will be re-downloaded and reinstalled when the upgraded container starts so your installation does't break.
Lastly, there's another new environment variable to enable container debugging: OTRS_DEBUG. This debugging is not very complex, it just enables bash debugging and installs some programs like telnet and dig to aid on troubleshooting.
I have been testing it on one of our installations and it's working very well, you should give it a try too !