Fix Dockerfile: use docker-cli instead of docker.io

docker.io on Debian Trixie only installs the daemon (dockerd), not the CLI.
The docker CLI is in the separate docker-cli package.

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

Dockerfile

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