Add Flutter app skeleton

scripts/flutter-docker.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ROOT_DIR="$(cd "$(dirname "$0")/.." && pwd)"
+IMAGE="${FLUTTER_DOCKER_IMAGE:-ghcr.io/cirruslabs/flutter:stable}"
+
+docker run --rm \
+  -u 0:0 \
+  -v "$ROOT_DIR:/workspace" \
+  -w /workspace/app \
+  "$IMAGE" \
+  sh -lc "flutter pub get && flutter $* && chown -R 1000:1000 /workspace/app"