chore(docker): add development environment with hot reload

Add dev stage to Dockerfile and startup script
Create docker-compose.dev.yml for local development
Update README with Docker dev mode instructions

.docker/dev-start.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+set -eu
+
+pnpm install --frozen-lockfile
+
+exec pnpm dev --host "${HOST:-0.0.0.0}" --port "${PORT:-20013}"