authenticate docker host

.trampolinerc

@@ -49,4 +49,19 @@ if [[ -z "${TRAMPOLINE_BUILD_FILE:-}" ]]; then
 fi
 
 # Secret Manager secrets.
+TRAMPOLINE_HOST=$(echo "${TRAMPOLINE_IMAGE}" | cut -d/ -f1)
+if [[ ! "${TRAMPOLINE_HOST}" =~ "gcr.io" ]]
+then
+  echo "TRAMPOLINE_HOST: ${TRAMPOLINE_HOST}"
+  echo "HOME: ${HOME}"
+  echo "${HOME}":/home
+  docker run \
+    --entrypoint=gcloud \
+    --env HOME=:/home \
+    --volume="${HOME}":/home \
+    "gcr.io/google.com/cloudsdktool/cloud-sdk" \
+    auth configure-docker "${TRAMPOLINE_HOST}"
+  echo "docker config:"
+  cat "${HOME}/.docker/config.json"
+fi
 source ${PROJECT_ROOT}/.kokoro/populate-secrets.sh