Add tags for nightly images

[tgross35]

E.g. rust-lang/rust:nightly-2024-02-20, so that it is easy to locate and pin specific images.

The output of rustc --version also includes a next version number (e.g. rustc 1.78.0-nightly), but just the date should probably suffice for this.

[sfackler]

This hasn't been done historically since it'll impose a pretty huge storage burden on Docker Hub/Github and I don't know if they're okay with that.

[tgross35]

Ah, yeah that would be significant. Maybe it would be possible to do something like tag a nightly every 2 weeks - that would be approximately 13GB/year if only the rust layer changes and not the dependencies

[tgross35]

#121 may also help slim down sizes even more if that hasn't been changed

[tyranron]

There is the mirrored instrumentisto/rust images preserving the date for nightly.

Previous issue: rust-lang/docker-rust-nightly#3