diff --git a/.github/workflows/build-docker.yml b/.github/workflows/build-docker.yml
index d97f5f05ede42..d6f7b3e46b35b 100644
--- a/.github/workflows/build-docker.yml
+++ b/.github/workflows/build-docker.yml
@@ -8,6 +8,13 @@ on:
       - 'tools/docker/config/*'
       - '.github/versions.sh'
       - '.github/workflows/build-docker.yml'
+  pull_request:
+    paths:
+      - 'tools/docker/Dockerfile'
+      - 'tools/docker/bin/run.sh'
+      - 'tools/docker/config/*'
+      - '.github/versions.sh'
+      - '.github/workflows/build-docker.yml'
 concurrency:
   group: build-docker-${{ github.event_name }}-${{ github.ref }}
   cancel-in-progress: true
@@ -19,11 +26,19 @@ jobs:
     permissions:
       packages: write
       contents: read
-    timeout-minutes: 20  # 2021-10-14: Test run took 5 minutes, but there's lots of downloading involved.
+    timeout-minutes: 60  # 2021-10-26: Build for arm64 is S-L-O-W. Sigh.
 
     steps:
       - uses: actions/checkout@v2
 
+      - name: Set up qemu
+        uses: docker/setup-qemu-action@v1
+        with:
+          platforms: arm64
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+
       - name: Log in to Docker Hub
         uses: docker/login-action@v1
         with:
@@ -46,16 +61,25 @@ jobs:
           echo "::set-output name=node-version::$NODE_VERSION"
           echo "::set-output name=pnpm-version::$PNPM_VERSION"
 
+          # We're not git-tagging for the env. Just tag all master builds as latest.
+          if [[ "$GITHUB_EVENT_NAME" == "push" ]]; then
+            echo "::set-output name=tags::type=raw,latest"
+            echo "::set-output name=images::automattic/jetpack-wordpress-dev%0Aghcr.io/automattic/jetpack-wordpress-dev"
+          elif [[ "$GITHUB_EVENT_NAME" == "pull_request" ]]; then
+            echo "::set-output name=tags::type=ref,event=pr"
+            echo "::set-output name=images::ghcr.io/automattic/jetpack-wordpress-dev"
+          else
+            echo "Unknown GITHUB_EVENT_NAME $GITHUB_EVENT_NAME"
+            exit 1
+          fi
+
       - name: Extract Docker metadata
         id: meta
         uses: docker/metadata-action@v3
         with:
-          # We're not git-tagging for the env. Just tag all builds as latest.
-          tags: |
-            type=raw,latest
-          images: |
-            automattic/jetpack-wordpress-dev
-            ghcr.io/automattic/jetpack-wordpress-dev
+          flavor: latest=false
+          tags: ${{ steps.buildargs.outputs.tags }}
+          images: ${{ steps.buildargs.outputs.images }}
           labels: |
             org.opencontainers.image.title=Jetpack Development Environment
             org.opencontainers.image.description=Unified environment for developing in the Jetpack Monorepo using Docker containers.
@@ -65,6 +89,7 @@ jobs:
         uses: docker/build-push-action@v2
         with:
           context: tools/docker
+          platforms: linux/amd64,linux/arm64
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}