[llvm] [libc++] Add a Github action to build libc++'s Docker images (PR #110020)

[via llvm-commits]

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@@ -0,0 +1,51 @@
+# This file defines an action that builds the various Docker images used to run
+# libc++ CI whenever modifications to those Docker files are pushed to `main`.
+#
+# The images are pushed to the LLVM package registry at https://github.com/orgs/llvm/packages
+# and tagged appropriately. The selection of which Docker image version is used by the libc++
+# CI nodes at any given point is controlled by the libc++ maintainers using a mechanism that
+# is not publicly visible for safety purposes.
+
+name: Build Docker images for libc++ CI
+
+permissions:
+  contents: read
+  packages: write
+
+on:
+  push:
+    branches:
+      - main
+    paths:
+      - 'libcxx/utils/ci/**'
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    if: github.repository_owner == 'llvm'
+
+    steps:
+    - uses: actions/checkout at v4
+
+    - name: Log in to GitHub Container Registry
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EricWF wrote:

Can we move the login after the build, that way we can't capture the docker credentials inside the build?

https://github.com/llvm/llvm-project/pull/110020