Investigate earthly for CI

[btkostner]

Turns out the Elixir repository uses https://github.com/earthly/earthly for CI and in GitHub actions. This would make a convenient way to run CI locally. We should investigate to see if we can get it running as fast as native GitHub actions and if it would be a worthy replacement.

[btkostner]

Errr, not the Elixir repository but the ecto-sql repo.

[Nezteb]

I also could have sworn that elixir-lang/elixir itself used Earthly at one point; maybe it was a brief experiment. 🤔

There are a few other "big" Elixir orgs that use Earthly though, Phoenix and Ecto being the most notable probably.