Update healthcheck to use APP_PORT environment variable

Replace hardcoded port 80 with APP_PORT variable for better flexibility.

docker-compose.yml

@@ -11,7 +11,7 @@ services:
         environment:
             TZ: ${APP_TIMEZONE}
         healthcheck:
-          test: [ "CMD", "curl", "-f", "http://localhost:80" ]
+          test: [ "CMD", "curl", "-f", "http://localhost:${APP_PORT:-80}" ]
           interval: 10s
           timeout: 5s
           retries: 5