Add healthcheck

Inquire http://127.0.0.1:3000/api/v1/comments/jNQXAC9IVRw every 30 seconds, mark Unhealthy when an error returns.
https://github.com/iv-org/documentation/pull/155
(I didn't put Autoheal in here.)

docker-compose.yml

@@ -35,6 +35,11 @@ services:
         full_refresh: false
         https_only: false
         domain:
+    healthcheck:
+      test: wget -nv --tries=1 --spider http://127.0.0.1:3000/api/v1/comments/jNQXAC9IVRw || exit 1
+      interval: 30s
+      timeout: 5s
+      retries: 2
     depends_on:
       - postgres