Fix .env.prod never reaching the puzzle relay containers

docker-compose's env_file loads .env directly into each container's
real environment - and Symfony's Dotenv never lets a .env.$APP_ENV
file override a variable the platform already set. Since .env.prod
was never in the env_file list, PUZZLE_RELAY_PUBLIC_URL kept resolving
to the dev default (ws://127.0.0.1:8091) in production instead of the
wss://dmtools.fvandenberg.nl/puzzle-ws/ override. Load .env.prod here
too, after .env/.env.local, so its values actually win.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docker-compose.yml

@@ -21,10 +21,17 @@ services:
       context: .
       dockerfile: docker/php/Dockerfile
     container_name: dmtools-php
+    # Real container env vars always beat Symfony's own .env.$APP_ENV cascade
+    # (that's the point of Dotenv - it never overrides a var the platform
+    # already set) - so .env.prod has to be loaded here too, or its values
+    # (e.g. PUZZLE_RELAY_PUBLIC_URL) never reach the app; the plain .env
+    # default silently wins instead. Order matters: later files win.
     env_file:
       - path: .env
       - path: .env.local
         required: false
+      - path: .env.prod
+        required: false
     volumes:
       - .:/var/www/html:cached
       - php_var:/var/www/html/var
@@ -43,6 +50,8 @@ services:
       - path: .env
       - path: .env.local
         required: false
+      - path: .env.prod
+        required: false
     volumes:
       - .:/var/www/html:cached
       - php_var:/var/www/html/var