Ensure that docker tag is legal (#46035)

dev/ci/docker_linux/docker_build.sh

@@ -5,6 +5,10 @@
 
 TAG="${CIRRUS_TAG:-latest}"
 
+# Convert "+" to "-" to make hotfix tags legal Docker tag names.
+# See https://docs.docker.com/engine/reference/commandline/tag/
+TAG=${TAG/+/-}
+
 # pull to make sure we are not rebuilding for nothing
 sudo docker pull "gcr.io/flutter-cirrus/build-flutter-image:$TAG"