[libcxx] Enable installing new runner binary on existing container

.github/workflows/libcxx-build-containers.yml

@@ -36,7 +36,9 @@ jobs:
 
     - name: Build the Linux builder image
       working-directory: libcxx/utils/ci
-      run: docker compose build actions-builder
+      run: |
+        docker compose build builder-base
+        docker compose build actions-builder
       env:
         TAG: ${{ github.sha }}
 
@@ -57,6 +59,7 @@ jobs:
       if: github.event_name == 'push'
       working-directory: libcxx/utils/ci
       run: |
+        docker compose push builder-base
         docker compose push actions-builder
       env:
         TAG: ${{ github.sha }}

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.
@@ -310,7 +311,7 @@ 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