Docker: Mount rust from image at build time instead of downloading it

[LaurentGoderre]

This ensures that rust is not left behind in the image

[tianon]

Because this is from= a stage, it would be mostly OK, but it's technically not officially supported in the DOI build system / dependency calculation, so adding it makes me a little nervous: https://github.com/docker-library/bashbrew/blob/f71d6ef63e1f5abe6d5cb06a62a3e5e42d68566e/cmd/bashbrew/docker.go#L96-L190

(For example, if we accept this, and then the FROM ... AS rust gets removed later, we won't necessarily remember the implications of that meaning that this line now implies from=rust:latest which will "just work" but not be accounted for correctly in our dependency calculations.)

[LaurentGoderre]

Would using a target that isn't matching an image solve this such as rust-build?

[thresheek]

Hmm, I think we can just re-use the "minimal" image instead of creating yet another "base" one. I'll try to give it a shot.

[thresheek]

I've pushed some commits that implement the "mount rust" idea, as well as removing build duplication into https://github.com/thresheek/unit/commits/optimize-docker-take2/. I can create another PR, or force-push to @LaurentGoderre's branch if @LaurentGoderre doesnt mind so we can review them here.

I've split the changes into reviewable items, so it should be easier to catch issues this way.

One thing I don't like is that the majority of "modules" don't really need a Rust image mounted since they won't be building any Rust code - the only users are minimal and wasm. I can live with it though if that's ok for Docker Library maintainers. We can probably also get rid of those with some more sed trickery and defines, but I'd rather leave it as is.

[LaurentGoderre]

@thresheek feel free to use my branch however you need!

[thresheek]

There we go. Please take a look.

[LaurentGoderre]

@thresheek LGTM!