Add Docker support and improve Instagram login flow

[dave-73]

Adds containerized execution with Docker Compose and improves login reliability.

Changes:

• Add Dockerfile and docker-compose.yml for isolated execution
• Run container as a non-root user and persist runtime data under ./data
• Ignore cookies, local data, and generated exports
• Use instagrapi for current Instagram login and 2FA support
• Add fallback for importing an authenticated Cookie header
• Add httpx[http2] dependency because the scraper enables HTTP/2
• Update README with Docker and authentication notes

Validation:

• python -m py_compile main.py login.py
• docker compose config
• docker compose build