Add step to log into GAR

Author: grahamalama

.github/workflows/build-publish.yaml

@@ -60,6 +60,14 @@ jobs:
           service_account: artifact-writer@${{ env.GCP_PROJECT_ID }}.iam.gserviceaccount.com
           workload_identity_provider: ${{ vars.GCPV2_GITHUB_WORKLOAD_IDENTITY_PROVIDER }}
 
+      - name: Login to GAR
+        uses: docker/login-action@v3
+        if: github.event_name != 'pull_request'
+        with:
+          registry: ${{ env.GAR_LOCATION }}-docker.pkg.dev
+          username: oauth2accesstoken
+          password: ${{ steps.gcp_auth.outputs.access_token }}
+
       - name: Login to Docker Hub
         if: github.event_name != 'pull_request'
         uses: docker/login-action@v3