Pre-populate cargo cache

[davidblewett]

The issue of the same name for the was closed due to the requirement of official images to be deterministic ( rust-lang/docker-rust#1 ).

It seems like it would make sense for the nightly version that doesn't have this restriction to have the local registry pre-populated with a snapshot of the state of the registry on that day. In my testing, a simple cargo search foo was enough to trigger a download. Maybe this could be new tag?

[sfackler]

Even though we're not required to make the builds here deterministic, I'd still like them to mirror the official images as much as possible.

[TriplEight]

anyways if you build without --no-cache flag, you'll get caches from the previous builds.