Reduce docker container size (+ some other improvements)

[flocke]

This pull requests aims to reduce the size of the Docker container uploaded to the registry.

Result:

This is mainly achieved by using a separate build container for the Rust build and then only copying the finished binary to the final container.

In addition to the size reduction some other optimizations are added:

• Simplify the user setup (only one command now)
• Remove gosu (the binary is directly executed as user)
• Remove the custom entrypoint script (apt-get update in entrypoint is bad practice, see https://stackoverflow.com/a/78804699)