Create healthcheck script for synapse-workers container (#11429)

The intent is to iterate through all the worker ports and only
report healthy when all are healthy, starting with the main process.

docker/conf-workers/healthcheck.sh.j2

@@ -0,0 +1,6 @@
+#!/bin/sh
+# This healthcheck script is designed to return OK when every 
+# host involved returns OK
+{%- for healthcheck_url in healthcheck_urls %}
+curl -fSs {{ healthcheck_url }} || exit 1
+{%- endfor %}