add healthcheck and fix shutdown order

Dockerfile

@@ -20,4 +20,5 @@ RUN npm run build
 # entry
 WORKDIR /usr/src/guppe
 EXPOSE 443 80
+HEALTHCHECK CMD /usr/bin/curl --fail http://localhost/ || exit 1
 CMD [ "node", "index.js" ]

index.js

@@ -225,10 +225,10 @@ client.connect()
       console.log('Guppe server listening on port ' + PORT_HTTPS)
     })
     onShutdown(async () => {
-      await client.close()
       await new Promise((resolve, reject) => {
         server.close(err => (err ? reject(err) : resolve()))
       })
+      await client.close()
       console.log('Guppe server closed')
     })
   })