Add docker-compose-plugin to Dockerfile

docker-cli alone doesn't include 'docker compose' subcommand.
Need docker-compose-plugin for 'docker compose -f' to work.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Dockerfile

@@ -4,11 +4,12 @@ FROM python:3.11-slim
 WORKDIR /app
 
 # Install system dependencies
-# - Docker CLI for docker executor (docker.io only has daemon, need docker-cli for CLI)
+# - Docker CLI + Compose plugin for docker executor
 # - curl for health checks
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
         docker-cli \
+        docker-compose-plugin \
         curl \
     && rm -rf /var/lib/apt/lists/*