Update Python base image, dependencies, add documentation (#14, #25)

[ssciolla]

This PR updates the Python base image in Dockerfile and the pip dependency versions; brings the documentation current with changes to the project; and makes other small changes to clean up and bring things in line with other T&L projects. Some additional details are provided in a task list below. The PR aims to resolve issues #14 and #25.

• [x] Update base image in Dockerfile to python:3.8-slim, add build-essential, and remove other Linux packages listed.
• [x] Update numpy, pandas, and psycopg2-binary versions in requirements.txt, removing sub-dependencies that don't need to be specified explicitly.
• [x] Update documentation with Overview and Development sections, including virtualenv and Docker instructions.
• [x] Tweak logging setup and config file loading in validate.py to ensure predictable, comprehensible log messages (also sys.exit(1) if config file isn't found).
• [x] Specify dtype in pd.Series constructor to suppress pandas warning.
• [x] Remove artifacts of legacy .env usage.
• [x] Make .gitignores in config and data empty, and only explicitly ignore things in root .gitignore.
• [x] Add .dockerignore.

[ssciolla]

Thanks, @zqian, for the review!