ci: parse smoke response as json

Jenkinsfile.backend-docker

@@ -181,7 +181,11 @@ pipeline {
                       $DOCKER_CMD logs "$SMOKE_APP" >&2
                       exit 1
                     fi
-                    grep -Fq "\"version\":\"$IMMUTABLE_TAG\"" artifacts/smoke-ping.json
+                    python3 - "$IMMUTABLE_TAG" <<'PY'
+import json, pathlib, sys
+payload = json.loads(pathlib.Path('artifacts/smoke-ping.json').read_text(encoding='utf-8'))
+assert payload.get('version') == sys.argv[1], payload
+PY
                     curl --fail --silent --show-error "http://127.0.0.1:$host_port/" \
                       | grep -Fq '<div id="app"></div>'
                 '''