Ensure releases can only occur on master

Author: Syriiin

Makefile

@@ -48,6 +48,7 @@ endif
 ifndef GITHUB_USERNAME
 	$(error GITHUB_USERNAME env var is not set)
 endif
+	@if [ $(shell git branch --show-current) != "master" ]; then echo "Error: This command can only be run on the master branch"; exit 1; fi;
 	echo $$GITHUB_PAT | docker login ghcr.io --username $$GITHUB_USERNAME --password-stdin
 	VERSION=$(VERSION) $(COMPOSE_PUBLISH) build
 	VERSION=$(VERSION) $(COMPOSE_PUBLISH) push difficalcy-osu difficalcy-taiko difficalcy-catch difficalcy-mania