Automerge: [libcxx] Enable installing new runner binary on existing container

This patch does some refactoring to enable installing a new GHA runner binary
into an existing libcxx image. We achieve this by pushing the base image to the
registry and enabling control over the base image used for building the actions
image. This will always build and push both images even if an existing image is
being used for the actions image, but this should not impact anything as the
SHAs are pinned everywhere and space/build time is not a large concern.

Reviewers: ldionne, EricWF, #reviewers-libcxx

Reviewed By: ldionne

Pull Request: llvm/llvm-project#148073

Author: boomanaiden154

libcxx/utils/ci/Dockerfile

@@ -38,6 +38,7 @@
 # If you're only looking to run the Docker image locally for debugging a
 # build bot, see the `run-buildbot-container` script located in this directory.
 
+ARG ACTIONS_BASE_IMAGE
 
 # HACK: We set the base image in the docker-compose file depending on the final target (buildkite vs github actions).
 # This means we have a much slower container build, but we can use the same Dockerfile for both targets.
@@ -309,7 +310,7 @@ CMD /opt/android/container-setup.sh && buildkite-agent start
 #
 # IMAGE: ghcr.io/libcxx/actions-builder.
 #
-FROM builder-base AS actions-builder
+FROM $ACTIONS_BASE_IMAGE AS actions-builder
 
 ARG GITHUB_RUNNER_VERSION
 

libcxx/utils/ci/docker-compose.yml

@@ -3,6 +3,16 @@ x-versions: &compiler_versions
   LLVM_HEAD_VERSION: 21
 
 services:
+  builder-base:
+    image: ghcr.io/llvm/libcxx-linux-builder-base:${TAG}
+    build:
+      context: .
+      dockerfile: Dockerfile
+      target: builder-base
+      args:
+        BASE_IMAGE: ubuntu:jammy
+        <<: *compiler_versions
+
   actions-builder:
     image: ghcr.io/llvm/libcxx-linux-builder:${TAG}
     build:
@@ -11,6 +21,7 @@ services:
       target: actions-builder
       args:
         BASE_IMAGE: ubuntu:jammy
+        ACTIONS_BASE_IMAGE: builder-base
         GITHUB_RUNNER_VERSION: "2.326.0"
         <<: *compiler_versions