Run containers on Mountain time (TZ=America/Denver or host /etc/localtime)

Images with zoneinfo get TZ=America/Denver; images without it mount the
host zone file read-only (TZ= would silently fall back to UTC there).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker-compose.yml

@@ -9,6 +9,7 @@ services:
       - ./data:/data
       - /etc/localtime:/etc/localtime:ro
     environment:
+      - TZ=America/Denver
       - DATABASE_URL=${DATABASE_URL}
       - ADMIN_TOKEN=${ADMIN_TOKEN}
       - SIGNUPS_ALLOWED=false