Allow libc++ image tag to be specified via enviroment variables.

This change is needed for changes I'm working on that allow
github workflows to build, push, and otherwise manage the container
images they use

libcxx/utils/ci/Dockerfile

@@ -294,6 +294,10 @@ CMD /opt/android/container-setup.sh && buildkite-agent start
 #
 FROM builder-base AS actions-builder
 
+# Install 'act' for running github actions locally. This provides an alternative to the run-buildbot script
+# while still providing reproducability.
+RUN curl -s https://raw.githubusercontent.com/nektos/act/master/install.sh | sudo bash
+
 WORKDIR /home/runner
 USER runner
 

libcxx/utils/ci/docker-compose.yml

@@ -6,7 +6,7 @@ x-versions: &compiler_versions
 
 services:
   buildkite-builder:
-    image: ghcr.io/libcxx/buildkite-builder:testing
+    image: ghcr.io/libcxx/buildkite-builder:${TAG:-latest}
     build:
       context: .
       dockerfile: Dockerfile
@@ -15,16 +15,16 @@ services:
         BASE_IMAGE: ubuntu:jammy
         <<: *compiler_versions
   actions-builder:
-    image: ghcr.io/libcxx/actions-builder:testing
+    image: ghcr.io/libcxx/actions-builder:${TAG:-latest}
     build:
       context: .
       dockerfile: Dockerfile
       target: actions-builder
       args:
-        BASE_IMAGE: ghcr.io/actions/actions-runner:latest
+        BASE_IMAGE: ghcr.io/actions/actions-runner:2.311.0
         <<: *compiler_versions
   android-buildkite-builder:
-    image: ghcr.io/libcxx/android-buildkite-builder:testing
+    image: ghcr.io/libcxx/android-buildkite-builder:${TAG:-latest}
     build:
       context: .
       dockerfile: Dockerfile