DEV: Update docker development binstubs

- Add `d/ember-cli`, and publish port 4200
- Remove `d/sidekiq`. Sidekiq is now started with the rails server
- Move all `docker exec` logic into a single place, so we have one place to set environment variable pass-throughs
- Use `exec` for all bash scripts, so that return statuses are passed back correctly
- Avoid using `bin/bash -c` unnecessarily, because it makes escaping arguments difficult

bin/docker/shell

@@ -1,2 +1,3 @@
 #!/bin/bash
-docker exec -it -u discourse:discourse discourse_dev /bin/bash
+
+exec "$(dirname "$0")/exec" /bin/bash "$@"