Add support for local secrets

[KevinJBoyer]

Ticket

n/a

Changes

• Provide an example docker compose override file
• Update .gitignore to ensure non-example override files aren't committed
• Update documentation (plus comment in local.env) to explain how to use it
• Move the docker compose files to inside the app folder
• Update the developer set up instructions

Context for reviewers

• This allows developers to set local secrets that are passed to their Docker container.

Testing

Follow the instructions in the documentation and verify the contents of your .env match what's available to the Docker container.