feat: Reduce image size with distroless image

[maweil]

As mentioned in #25, I have worked on a smaller mumble image for personal use. The PR is not yet ready for merging as there are still some TODOs to address. I initially built this a longer time ago so I still need to check whether no functionality is lost by this PR.

Also please note that I'm still learning rust and do not claim this is the most elegant way to replace entrypoint.sh. I did this mainly as a learning opportunity.

Main Differences

• Replace entrypoint script with rust binary
  • To be able to run without a shell
• Copy over only needed shared libs

Open TODOs

• [ ] Add shared libs for other database drivers than SQLite
• [ ] Test build with current final (1.4.x), not only 1.5.x
• [ ] Check that all options from entrypoint.sh are implemented
• [ ] Fix build on ARM64