Make Observatory staging deploy rerunnable (#155)

.github/workflows/gcp-observatory-read-adapter.yml

@@ -74,12 +74,21 @@ jobs:
           set -euo pipefail
           tag="${GITHUB_SHA::12}"
           image="${REGION}-docker.pkg.dev/${PROJECT_ID}/${REPOSITORY}/${IMAGE_NAME}:${tag}"
+          digest="$(gcloud artifacts docker images describe "${image}" \
+            --project="${PROJECT_ID}" \
+            --format='value(image_summary.digest)' 2>/dev/null || true)"
+          if [[ -n "${digest}" ]]; then
+            docker pull "${image}"
+          else
             docker build -f Dockerfile.observatory-read-adapter -t "${image}" .
+          fi
           docker run --rm --env PYTHONPYCACHEPREFIX=/tmp/pycache \
             "${image}" python -m compileall -q /app/observatory_read_adapter
           docker run --rm "${image}" python -c 'import observatory_read_adapter; print("adapter-import-ok")'
+          if [[ -z "${digest}" ]]; then
             docker push "${image}" | tee docker-push.log
             digest="$(awk '/digest: sha256:/ {print $3}' docker-push.log | tail -1)"
+          fi
           test -n "${digest}"
           image_ref="${image}@${digest}"
           printf 'image_ref=%s\n' "${image_ref}" >> "${GITHUB_OUTPUT}"