entrypoint: Only execute main function if the script is not sourced

If entrypoint.sh is sourced from elsewhere we don't execute the main
function, so custom entrypoint scripts can use the helper functions
directly.

.template.Debian/docker-entrypoint.sh

@@ -322,5 +322,8 @@ docker_main() {
 	fi
 	exec "$@"
 }
-
-docker_main "$@"
+# This checks if the script has been sourced from elsewhere.
+# If so we don't perform any further actions
+if [ ! "${FUNCNAME[${#FUNCNAME[@]} - 1]}" = 'source' ]; then
+	docker_main "$@"
+fi