Thanks Scobie!
I was just about to write down what my problems are, as I am "playing" around for some hours now with Docker.
I was able to install Docker and the MC24 package, which really is simple. After the installation, an URL is popping up eventually that I can klick to be forwarded to a new browser tab. I don't know what triggers that URL, but sometimes it seems as if it needs a little push by closing and re-opening the Container Station window. In the new browser tab a JRiver panel shows up where I could access my music if Media Center would know where my music files were.
Within QNAPs Container Station there is a console field which I think was the place to type in certain commands to set-up MC. This is not possible here but propably should be possible and only is a problem with my computer set-up. I tried this on two browser engines, Safari and Waterfox, which is a Firefox derivate I am using.
I am trying a VNC app with the URL I mentioned above, but it stops connecting to Media Center after some while, "the connection closed unexpectedly". Same goes with a VNC app I tried on my iPad.
This morning I was able to sucessfully connect to Media Center and I was able to add my license. But I was not able to find the directory with my audio files. So I quit the app to do some more research on the file structure of Linux. My guess at the moment is, that I somehow have to mount the file path within Docker to the MC app. But I do not know how to do this (see above). Anyway I quit the VNC app and was re-starting the MC Docker package too. When I re-opened the VNC app, the bookmark I made for my last session was still there, but the link was broken and I couldn't access MC since then.
So, that's where I am now. It's most likely my inexperience that i haven't found out how things work. At the moment I am learning a lot about Docker from internet articles and videos and I'm sure will succeed at some time!
Have a great 2019!