Add Docker-GHCR auth step

Signed-off-by: Martijn Stevenson <[email protected]>

Author: martijneken

.github/workflows/test.yml

@@ -46,6 +46,14 @@ jobs:
     steps:
     - uses: actions/checkout@v2
 
+    # Needed for s390x test which runs on a GHCR Docker Ubuntu image.
+    - name: Login to GitHub Container Registry
+      uses: docker/login-action@v3
+      with:
+        registry: ghcr.io
+        username: ${{ github.actor }}
+        password: ${{ secrets.GITHUB_TOKEN }}
+
     - name: Bazel cache
       uses: PiotrSikora/[email protected]
       with: