Move Docker files into .devcontainer

[machikoyasuda]

closes #91

What this PR does

• Moves Docker and related files into .devcontainer, and updates files accordingly
• Updates docs with instruction to cd into .devcontainer

How to test

• Run cd .devcontainer and then docker compose --no-cache build server and then docker compose --no-cache build dev
• Reopen the repository in VS Code Remote Container. Ensure that pre-commit is installed and running.
• Run the app in Run and Debug mode
• Run python setup.py - Should run successfully
• Run coverage run -m pytest -m databasetest - All tests should pass
• Run source tests/.env.settingstest and then coverage run -m pytest -m settingstest - All tests should pass
• Test that editing files locally works

[thekaveman]

Looking at this.

And it's helping me realize that I messed something up in cal-itp/benefits#598 - the docker compose ... commands don't work unless you have an .env file in the directory, but the instructions / thinking was that the file is optional and only needed for overrides.

EDIT: tracking this in cal-itp/benefits#626