fix: pass PORT env var to app container

Apps follow the Heroku convention of binding to $PORT at runtime.
Without --env PORT=$PORT, containers use their default port which
doesn't match what Caddy is configured to dial, causing 502s.

builder/build.sh

@@ -100,6 +100,7 @@ docker run --detach \
     --name "$CONTAINER_NAME" \
     --network hiy-net \
     "${ENV_FILE_ARG[@]+"${ENV_FILE_ARG[@]}"}" \
+    --env "PORT=${PORT}" \
     --expose "$PORT" \
     --label "hiy.app=${APP_ID}" \
     --label "hiy.port=${PORT}" \