Docker tweaks

[jberthold]

A few improvements to make the docker containers start faster (fixuid goes through loads of files and takes >3min to start - those were probably the CI problems with permissions that we observed when using docker exec)

• delete a large number of files that aren't needed to work with the docker image

• relocate rustup installation out of /home/kmir

• only run fixuid on the kmir home directory

• change RUN command to use json style (avoids warning messages from the plugin in CI)

Before this, it took very long to start the container , now the start time is down to 9 seconds on my machine.
We could try to not chmod in $HOME/.cache (where a lot of files from the K definition are sitting) but that might break the run.

[ehildenb]

Overall this looks fine to me, but I don't know what this docker image is used for. So I'll approve, but if it's something you're not sure of you should maybe wait for Freeman's approval.