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

[Aiden Grossman 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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boomanaiden154 wrote:

We probably want a `workflow_dispatch` here too for cases like rebuilding the container with the latest packages in the Ubuntu repos if there's something like a security update.

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