Add healthcheck to Dockerfile

This allows for safer zero-downtime upgrades.

Dockerfile

@@ -57,6 +57,8 @@ USER rails:rails
 # Entrypoint prepares the database.
 ENTRYPOINT ["/rails/bin/docker-entrypoint"]
 
+HEALTHCHECK --timeout=5s --start-period=20s CMD curl -sS --fail http://localhost:3000/up || exit 1
+
 # Start the server by default, this can be overwritten at runtime
 EXPOSE 3000
 CMD ["./bin/rails", "server"]