fix(docker-library): override the default branch

The Docker Official Images library defaults to `master`, but this
repository's default branch is called `main`.

Author: niheconomoum

generate-stackbrew-library.sh

@@ -71,6 +71,7 @@ cat <<-EOH
 
 Maintainers: Matthew X. Economou <[email protected]> (@niheconomoum)
 GitRepo: https://github.com/IdentityPython/satosa-docker.git
+GitFetch: refs/heads/main
 EOH
 
 # prints "$2$1$3$1...$N"