Add curl for basic healthcheck

[dvanderveer]

Description

This change adds curl to the lemmy-ui container for basic Docker healthcheck purposes. The lemmy.world admins would like this as it can give more immediate feedback when performing site maintenance.

Before

Without the change, any Docker healthcheck using curl will fail because the the command does not exist inside the container:

$ grep -A2 lemmy-ui docker-compose.yml
  lemmy-ui:
    image: dessalines/lemmy-ui:0.18.5
    healthcheck:
      test: curl -f http://127.0.0.1:1234 || exit 1

After

With the change, a simple curl healthcheck shows the lemmy-ui container as healthy when correctly configured:

$ grep -A5 lemmy-ui docker-compose.yml
  lemmy-ui:
    image: dvlemmyui
    healthcheck:
      test: curl -f http://127.0.0.1:1234 || exit 1
    environment:
      - LEMMY_UI_LEMMY_INTERNAL_HOST=lemmy:8536
$ docker ps | grep lemmy-ui
5691ba6273d2   dvlemmyui                 "docker-entrypoint.s…"   About a minute ago   Up About a minute (healthy)   127.0.0.1:1235->1234/tcp   selfhost_lemmy-ui_1

And a misconfigured lemmy-ui container shows as unhealthy:

$ grep -A5 lemmy-ui docker-compose.yml
  lemmy-ui:
    image: dvlemmyui
    healthcheck:
      test: curl -f http://127.0.0.1:1234 || exit 1
    environment:
      - LEMMY_UI_LEMMY_INTERNAL_HOST=doesnt.exist:8536
$ docker ps | grep lemmy-ui
090161ee9d14   dvlemmyui                 "docker-entrypoint.s…"   About a minute ago   Up About a minute (unhealthy)   127.0.0.1:1235->1234/tcp   selfhost_lemmy-ui_1