Use a unique name for the Docker image used by CI.

[Arnavion]

This allows testing multiple targets in parallel on the same host machine.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (or someone else) soon.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[Arnavion]

I needed this because I had to run CI locally for #3422 instead of having to wait for someone to approve it, and my machine has enough CPU and RAM to save time by running every target in parallel. It doesn't hurt to always do it even when running only one target as CI does, so I'm upstreaming it.

[JohnTitor]

Makes sense, thanks! @bors r+

[bors]

📌 Commit 90bd662 has been approved by JohnTitor

It is now in the queue for this repository.

[bors]

⌛ Testing commit 90bd662 with merge d0ce87b...

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-12, checks-cirrus-freebsd-13, checks-cirrus-freebsd-14
Approved by: JohnTitor
Pushing d0ce87b to main...