Ensure releases can only occur on master

Author: Syriiin

Makefile

@@ -47,6 +47,9 @@ ifndef GITHUB_PAT
 endif
 ifndef GITHUB_USERNAME
 	$(error GITHUB_USERNAME env var is not set)
+endif
+ifneq "$(shell git branch --show-current)" "master"
+	$(error This command can only be run on the master branch)
 endif
 	echo $$GITHUB_PAT | docker login ghcr.io --username $$GITHUB_USERNAME --password-stdin
 	VERSION=$(VERSION) $(COMPOSE_PUBLISH) build