Fix a bug in docker_push.sh, and indent in .cirrus.yml (#20308)

.cirrus.yml

@@ -121,8 +121,8 @@ task:
   osx_instance:
     image: high-sierra-xcode-9.4.1
   git_fetch_script:
-  - git fetch origin
-  - git fetch origin master # To set FETCH_HEAD
+    - git fetch origin
+    - git fetch origin master # To set FETCH_HEAD
   setup_script:
     - bin/flutter config --no-analytics
     - bin/flutter update-packages

dev/ci/docker_linux/docker_push.sh

@@ -2,5 +2,5 @@
 
 TAG="${CIRRUS_TAG:-latest}"
 
-docker push "gcr.io/flutter-cirrus/build-flutter-image::$TAG"
+docker push "gcr.io/flutter-cirrus/build-flutter-image:$TAG"