Add a CI job to build a docker image with the compiler installed

[arichardson]

This takes a bit over an hour to run with the targets limited to RISC-V+Host and it doesn't run the tests yet.
This should allow reusing the compiler in other GitHub actions.

Example run: https://github.com/arichardson/llvm-project/actions/runs/19867025000/job/56932513168 and the image from that can be fetched as ghcr.io/arichardson/llvm-project:github-actions-test

[arichardson]

Will drop the second commit adding this branch before merging.

[arichardson]

Hmm now it's failing with System.IO.IOException: No space left on device : '/home/runner/actions-runner/cached/_diag/Worker_20251202-234553-utc.log'

[arichardson]

Looks like this succeeded in 2 hours - I wonder if we should add running the tests?

[jrtc27]

Looks like this succeeded in 2 hours - I wonder if we should add running the tests?

How long does it take? Presumably the reason to do it is, although Jenkins will run the tests, that won't stop the GitHub Actions job from publishing the image if they're failing? (Ignoring the rare "Jenkins passes but GitHub fails" case)

[arichardson]

Looks like this succeeded in 2 hours - I wonder if we should add running the tests?

How long does it take? Presumably the reason to do it is, although Jenkins will run the tests, that won't stop the GitHub Actions job from publishing the image if they're failing? (Ignoring the rare "Jenkins passes but GitHub fails" case)

I've added the tests now. I think not publishing the image on failing tests is important so even if it takes 4 hours that's worthwhile. How do you feel about building+testing all targets vs just RISCV;X86;AArch64;Mips;ARM?

[github-actions]

Test Results

91 734 tests   88 548 ✅  2h 34m 26s ⏱️
     7 suites   3 186 💤
     1 files         0 ❌

Results for commit 641dbf6.