fix(ci): Include Dockerfile hash in Playwright image tag

The image tag was only based on the Playwright version, so changes to
the Dockerfile (e.g. adding yarn, system deps) didn't trigger a rebuild.

Tag is now `v<playwright-version>-<8-char-dockerfile-hash>`, e.g.
`v1.56.0-a1b2c3d4`. Any change to the Dockerfile produces a new tag,
which misses the GHCR cache and triggers a fresh build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mydea

.github/actions/ensure-playwright-image/action.yml

@@ -66,10 +66,11 @@ runs:
 
     - name: Set image reference
       id: image
-      run:
-        echo "ref=ghcr.io/${{ github.repository }}/playwright:v${{ steps.playwright.outputs.version }}" >>
-        $GITHUB_OUTPUT
       shell: bash
+      run: |
+        DOCKERFILE_HASH=$(sha256sum .github/docker/playwright.Dockerfile | cut -c1-8)
+        TAG="v${{ steps.playwright.outputs.version }}-${DOCKERFILE_HASH}"
+        echo "ref=ghcr.io/${{ github.repository }}/playwright:${TAG}" >> $GITHUB_OUTPUT
 
     - name: Check if image already exists
       id: check