* 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.
