[llvm] [workflows] Build a container for running CI on github actions (PR #75286)

[Tom Stellard via llvm-commits]

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+name: Build CI Container
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+permissions:
+  contents: read
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+on:
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tstellar wrote:

I considered doing this, but decided not to, because I didn't want just anyone to be able to rebuild the container.  One option would be to add a workflow_dispatch, but with a user/team check.

https://github.com/llvm/llvm-project/pull/75286