Docker cleanups

- Put docker-compose.yml into version control, no need to make this an example
  file and complicate things.
- Start the docker dev-env osem on port 3000. Just as our documentation says
  and how we do it in vagrant.
- hardcode CONTAINER_USERID so you don't have to have an
  docker-compose.override.yml if your UID is 1000.
- Rename web service/container to osem for clarity in the logs
- Keep assets also in a volume in production
- warn a bit more prominently about docker-compose.override.yml

.gitignore

@@ -35,7 +35,6 @@ pickle-email-*.html
 .env.development
 .env.test
 .env.local
-docker-compose.yml
 docker-compose.override.yml
 .DS_Store
 .byebug_history