docker: make relay opt-in via compose profile

Running a local relay stopped being the common case — most of the
time we're developing against a real one. Move the relay service
behind a `relay` profile so plain `docker compose up` only brings up
the app. Use `docker compose --profile relay up` when a local relay
is wanted.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -9,8 +9,12 @@ services:
       - ./public:/var/www/html/public
       - ./config.php:/var/www/html/config.php
 
+  # Relay is opt-in via the `relay` profile. Plain `docker compose up`
+  # starts just the app; use `docker compose --profile relay up` when
+  # you want to develop against a local relay instead of a remote one.
   relay:
     build: ./relay
+    profiles: ["relay"]
     ports:
       - "8081:80"
     volumes: