Authenticate against ghcr.io

This change adds a login step that authenticates the runner to
ghcr.io. This allows whomever triggers the action to use github
packages as a destination for the container images.

Signed-off-by: Gabriel Adrian Samfira <gsamfira@cloudbasesolutions.com>

.github/workflows/build-test-images.yml

@@ -143,6 +143,13 @@ jobs:
           service_account_key: ${{ secrets.GCP_SA_KEY }}
           export_default_credentials: true
 
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v1
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
       - name: Build and push images
         shell: bash
         run: |