Dockerfile: Size optimisations

[Tokarak]

• switch to latest-rust-slim
  Motivation: cut down on download sizes
  TODO:

• Remove git binary reference in bot/build.rs. Use crate built instead.

• Use https://github.com/slimtoolkit/slim

[mattfbacon]

How is using built related to this? If the problem is using the git binary I'd rather just do std::fs::read_to_string(".git/refs/heads/main").unwrap(), building an entire alternative git client seems like bloat.

[mattfbacon]

So I agree with these suggestions in principle but I don't actually use docker so I can't make any progress with this issue. What would you like to do with this?

[Tokarak]

I'm not sure I have time to work on this unfortunately. Status is as-is: easiest thing, probably, is to Rustify the build process so that rust-slim can be used.