Can't manage to add redmine plugins into the Redmine 5.1.2 image

[MangeChats]

Hello
I am a new user of this image, and managed to run it using the information given in the directives found on
https://hub.docker.com/_/redmine (which is very good news)
The pre-existing database from an old redmine installation is correctly handled with auto migration (even better).
This was really easy to do, and thank you for the good work.
But then I had some difficulties I think should not resist to your expert eyes...

First I think it would be interesting to update the paragraph concerning
Environment Variables:
I am using a mysql container for the database, and according to the container logs at startup, the
documented variables REDMINE_DB_MYSQL, REDMINE_DB_POSTGRES, and REDMINE_DB_SQLSERVER
are obsolete and should be replaced using REDMINE_DB_TYPE and REDMINE_DB_HOST
(I suppose this is really easy to improve)

Anyway I managed to get it, but I need help on the proper way to add Remine Plugins.
I managed to mount a docker volume into /usr/src/redmine/plugins
and when I add some plugins (I tested 3 different plugins) into this directory they are detected,
the plugin update is started but rapidly fails
with a container message of the type

LoadError: cannot load such file -- redmine_ckeditor (LoadError)

Maybe this plugins supposed to be Redmine 5.0 compatible is not 5.1 compatible ?
But all the tested pluging have the same problem...

Any suggestion would be appreciated, I spent several days on this point

[MangeChats]

I start to suspect I don't have the correct version of
Redmine X CKEditor which is supposed to be compatible with Redmine 5.0.X
I try to download from links provided in https://www.redmine.org/plugins/redmine-x-ck-editor-free
but they seem to be broken.

Just to be sure I tried with a container based on Redmine 5.0.8
and it still returns the same error message in the container logs
LoadError: cannot load such file -- redmine_ckeditor (LoadError)