Separate govuk-chat processes to individual services

This change resolves an issue where govuk-chat-app and govuk-chat-worker
both ran a sidekiq process, due to a worker process being registered on
Procfile.dev [1].

The approach taken is to move the processes of Procfile.dev to each be
individual services in docker compose, which has led to the creation of
the govuk-chat-css service that runs the dartsass compilation process.

This allows each process to have individual logs and resolves an issue
where it was previously very difficult to isolate the app logs as they
were mushed in with CSS.

Alternatives considered were:

1. Remove sidekiq from the Procfile.dev and have web / css handled by
   govuk-chat-app and worker be separate, this is rejected as there
   doesn't seem to be a logical reason - besides existing precedent -
   why we'd have have 2 of the 3 processes for app.
   Separate out CSS and web to be separate processes so there is
2. Run bin/dev on govuk-chat-app and have that cover all processes. This was
   the first approach tried [2] however after discussions with
   colleagues we agreed it felt strange to have 3 processes running for
   one container.

[1]: https://github.com/alphagov/govuk-chat/blob/e1d33470985f8a6c3690538b949bd36961e9bd57/Procfile.dev#L3
[2]: #735

projects/govuk-chat/docker-compose.yml

@@ -30,6 +30,7 @@ services:
   govuk-chat-app:
     <<: *govuk-chat
     depends_on:
+      - govuk-chat-css
       - govuk-chat-worker
       - nginx-proxy
       - postgres-13
@@ -41,7 +42,11 @@ services:
       BINDING: 0.0.0.0
     expose:
       - "3000"
-    command: bin/dev
+    command: bin/dev web
+
+  govuk-chat-css:
+    <<: *govuk-chat
+    command: bin/dev css
 
   govuk-chat-worker:
     <<: *govuk-chat
@@ -51,4 +56,4 @@ services:
     environment:
       REDIS_URL: redis://redis
       DATABASE_URL: "postgresql://postgres@postgres-13/govuk-chat"
-    command: bundle exec sidekiq -C ./config/sidekiq.yml
+    command: bin/dev worker