Windows Container and Docker already useful?

I was able to install it Windows Server 2016 TP5

I highly recommend using the newly released RTM preview. It’s much faster and more enjoyable. Details:

Does this mean, that in the future, you have to repack your image for every Windows Version

No. Compatibility was not a focus for the pre-RTM tech previews, so that’s why you’re seeing the error. Going forward post RTM, I think the major version just has to match (although Microsoft may not support all version combinations unless you’re using Hyper-V isolation).

the nanoserver (…) hangs when starting

Run Windows Update on Windows 10: https://github.com/Microsoft/Virtualization-Documentation/issues/358#issuecomment-249754005

Will I be able to use the same Images for Windows 10 and Windows Server 2016 anyway

Yes, Windows 10 only runs Windows Server based containers.

The windowsservercore is far too huge with 8 GB

Please see my response here: A couple of general questions regarding containers - #2 by friism