Add Docker Compose env detection

[Pinak-Datta]

envgap should understand common Docker Compose environment patterns so it can explain local-vs-container config drift.

Useful first scope:

• parse compose.yml, docker-compose.yml, and docker-compose.yaml
• detect environment: keys
• detect env_file: entries
• report variables used by Docker but missing from .env.example
• avoid reading secrets from unrelated files

This should stay diagnostic: envgap should explain what it found, not mutate compose files.