Add Dockerfile for containerized data generation

[midwire]

Summary

• Adds Dockerfile, entrypoint script, and .dockerignore so users can build and run the gem in a container without installing Ruby locally
• Container produces SQLite database and CSV files, retrieved via a mounted volume
• Supports optional COUNTRY env var to process a single country (validates 2-letter uppercase code)

Closes #20

Test Plan

• Docker image builds successfully
• docker run --rm -v $(pwd)/output:/output -e COUNTRY=AD free_zipcode_data produces SQLite DB + CSVs
• SQLite database contains expected data (7 zipcodes for Andorra)
• Invalid COUNTRY value rejected with clear error message
• All existing tests pass (117 examples, 0 failures)
• Rubocop clean (0 offenses)