diff --git a/.github/workflows/docker.yml b/.github/workflows/docker.yml
index 124b17458f45..b0582749dc4c 100644
--- a/.github/workflows/docker.yml
+++ b/.github/workflows/docker.yml
@@ -63,3 +63,69 @@ jobs:
           tags: "matrixdotorg/synapse:${{ steps.set-tag.outputs.tag }}"
           file: "docker/Dockerfile"
           platforms: linux/amd64,linux/arm64
+
+  build_workers_test:
+    runs-on: ubuntu-latest
+
+    # The worker test image depends on the base image, so we must build the base
+    # first.
+    needs: build
+
+    permissions:
+      packages: write
+      contents: read
+
+    steps:
+      - name: Set up QEMU
+        id: qemu
+        uses: docker/setup-qemu-action@v1
+        with:
+          platforms: arm64
+
+      - name: Set up Docker Buildx
+        id: buildx
+        uses: docker/setup-buildx-action@v1
+
+      - name: Inspect builder
+        run: docker buildx inspect
+
+      - name: Login to GitHub Container Registry (for worker-testing-only image)
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      # This must match EXACTLY the one in the `build` pipeline.
+      - name: Calculate docker image tag
+        id: set-tag
+        run: |
+          case "${GITHUB_REF}" in
+              refs/heads/develop)
+                  tag=develop
+                  ;;
+              refs/heads/master|refs/heads/main)
+                  tag=latest
+                  ;;
+              refs/tags/*)
+                  tag=${GITHUB_REF#refs/tags/}
+                  ;;
+              *)
+                  tag=${GITHUB_SHA}
+                  ;;
+          esac
+          echo "::set-output name=tag::$tag"
+
+      # This image is solely intended to be used for automated test tools,
+      # such as mx-tester.
+      - name: Build and push worker-testing-only image for all platforms
+        uses: docker/build-push-action@v2
+        with:
+          push: true
+          build-args: |
+            "base_version=${{ steps.set-tag.outputs.tag }}"
+          labels: "gitsha1=${{ github.sha }}"
+          tags: "ghcr.io/matrix-org/synapse-workers-testing-only:${{ steps.set-tag.outputs.tag }}"
+          file: "docker/Dockerfile-workers"
+          platforms: linux/amd64,linux/arm64
diff --git a/changelog.d/11852.misc b/changelog.d/11852.misc
new file mode 100644
index 000000000000..11abe3614feb
--- /dev/null
+++ b/changelog.d/11852.misc
@@ -0,0 +1 @@
+Build Docker images for using worker-mode Synapse in automated test tools.
\ No newline at end of file
diff --git a/docker/Dockerfile-workers b/docker/Dockerfile-workers
index 46f2e17382db..53f59e93625f 100644
--- a/docker/Dockerfile-workers
+++ b/docker/Dockerfile-workers
@@ -1,5 +1,8 @@
+ARG base_version=latest
+
 # Inherit from the official Synapse docker image
-FROM matrixdotorg/synapse
+
+FROM matrixdotorg/synapse:$base_version
 
 # Install deps
 RUN apt-get update