Use a docker entrypoint to perform environment setup instead of an action

[tlater-famedly]

Depend on famedly/rust-container#11. This only introduces one commit on top of #44, review that one if you're curious.

This significantly simplifies the env setup (and modernizes it a bit, we have a lot of references to Gitlab still lying around), making this repository more robust to internal changes (since GitHub actions don't have a concept of dependencies, not even within the same repository).

Of course, this won't work for workflows in which we intend to run on bare metal instead of a container. This is primarily a problem when we want to launch additional containers; I believe that we can cover that use case with docker-in-docker instead, though, which I think is worth it.

Right now this is a draft so that I can test that last assertion there, feasibility all depends on that.

[tlater-famedly]

I believe that we can't just rug-pull these in practice, we should probably make this a v2 branch if it happens.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.44%. Comparing base (31e5a07) to head (184486e).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #45   +/-   ##
=======================================
  Coverage   44.44%   44.44%           
=======================================
  Files           1        1           
  Lines           9        9           
=======================================
  Hits            4        4           
  Misses          5        5           

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