Fix docker tag during upload as well (#46252)

In https://github.com/flutter/flutter/pull/46035, we fixed the docker
tag to be a legal tag during build.  This PR makes the corresponding
change to the tag during upload.  Without this, we build the right tag,
but then we try to upload the wrong tag.

dev/ci/docker_linux/docker_push.sh

@@ -5,5 +5,8 @@
 
 TAG="${CIRRUS_TAG:-latest}"
 
-sudo docker push "gcr.io/flutter-cirrus/build-flutter-image:$TAG"
+# Convert "+" to "-" to make hotfix tags legal Docker tag names.
+# See https://docs.docker.com/engine/reference/commandline/tag/
+TAG=${TAG/+/-}
 
+sudo docker push "gcr.io/flutter-cirrus/build-flutter-image:$TAG"