gollum Rockon - harmonize with main latest image releases

[Hooverdan96]

as a continuation of #410 fulfilled via #411 further update gollum Rockon to pull the "latest" docker image (gollum made the decision to use the master as the tag for the latest release and bugfixes as opposed to the latest tag).

With version 6.x permission changes have been introduced that require the gollum share to be set up with permissions for a user/group that corresponds to the UID/GIDs of 1000:1000. This will be added to the description (like in the 2FAAuth Rockon) as well as mentioned in the description for the volume tag.