We aren't using gravatar or other 3rd party avatar services, and we don't want the default avatars either as they can give the same color to different users, making them look like the same user on first glance.
* Use the redmine:6.1.3 image from upstream (the sysmocom registry image was a workaround for 5.1.11).
* Install nkf with native extensions (needs gcc, libc6-dev, make) for the redmine_openid_provider plugin. This was not needed for the redmine 5 container, it was probably bundled there already.
* Upgrade "activerecord-session_store" to version 2.2.0, which is compatible with redmine 6.
