Load .env directly via env_file so DOMAIN_SUFFIX reaches containers

Using compose-level ${DOMAIN_SUFFIX} substitution only works when docker
compose is run from the same directory as the .env file. env_file loads
the file relative to the compose file, so it works regardless of CWD.

infra/docker-compose.yml

@@ -28,13 +28,15 @@ services:
       - hiy-data:/data
       # Mount the builder script so edits take effect without rebuilding.
       - ../builder:/app/builder:ro
+    env_file:
+      - path: .env
+        required: false
     environment:
       HIY_DATA_DIR: /data
       HIY_ADDR: 0.0.0.0:3000
       HIY_BUILD_SCRIPT: /app/builder/build.sh
       CADDY_API_URL: http://caddy:2019
       DOCKER_HOST: tcp://docker-proxy:2375
-      DOMAIN_SUFFIX: ${DOMAIN_SUFFIX:-localhost}
       RUST_LOG: hiy_server=debug,tower_http=info
     depends_on:
       caddy:
@@ -53,8 +55,9 @@ services:
       - "80:80"
       - "443:443"
       - "2019:2019"   # admin API
-    environment:
-      DOMAIN_SUFFIX: ${DOMAIN_SUFFIX:-localhost}
+    env_file:
+      - path: .env
+        required: false
     volumes:
       - ../proxy/Caddyfile:/etc/caddy/Caddyfile:ro
       - caddy-data:/data