[DRAFT] Optimise Docker Caching

[geektype]

Addresses rust-lang/infra-team#103.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Kobzol (or someone else) some time within the next two weeks.

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[geektype]

r? ghost

[rustbot]

Failed to set assignee to ghost: invalid assignee

Note: Only org members with at least the repository "read" role, users with write permissions, or people who have commented on the PR may be assigned.

[matthiaskrgr]

?

[geektype]

This is just a dummy commit.

Going to mess around with the CI so gonna use this PR to trigger builds. Let me know if there is a better way (this does feel hacky)

[Kobzol]

Docker caching isn't used at all on PR CI workflows. It is only used on master merges, where CI has credentials to actually access the cache. This can be tested only using a try build (that we can run for you manually), or (ideally), on your own fork of rust-lang/rust.

[geektype]

Makes sense. I will close this PR in that case!