docker: shift HEALTHCHECK to helper

Dockerfile

@@ -13,4 +13,4 @@ USER 1000:1000
 ENTRYPOINT ["/bin/sh", "/app/morss-helper"]
 CMD ["run"]
 
-HEALTHCHECK CMD python -m morss.crawler http://localhost:${PORT:-8000}/ > /dev/null 2>&1
+HEALTHCHECK CMD ["check"]

morss-helper

@@ -12,6 +12,9 @@ elif [ "$@" = "reload" ]; then
 	kill -s WINCH $pid
 	kill -s TERM $pid
 
+elif [ "$@" = "check" ]; then
+	python -m morss.crawler http://localhost:${PORT:-8000}/ > /dev/null 2>&1
+
 else
 	morss $@