feat(docker): add minimal support (ai implementation)

[rcmorano]

Changes:

• Dockerfile: Multi-stage build (builder/runtime) with Python 3.12-slim, non-root user security, optional health check
• docker-compose.yml: Service config with volume mounts for archive/cache, environment variables, port 5000 mapping
• README.md: Added Docker section with prerequisites, quick start, configuration guide, background execution, and stopping instructions

Key Features:

• Small image size via multi-stage build
• Non-root user for security
• Health check support
• Volume persistence for cache
• Easy configuration via docker-compose.yml and environment variables

Usage:

# Copy an export to $REPO_DIR/slack-export.zip
docker-compose up --build
# Open http://localhost:5000

[hfaran]

This seems prone to breaking over time. Is there no better way to install the dependencies properly?