Introduce Docker Compose env file

[istride]

The current setup instructions require environment variables to be change in the the Docker Compose file, but this is very error-prone. An env file is recommended for managing environment variables.

An example file ('env.example') is provided, with default values, which should be copied as '.env' in the same directory as the Docker Compose file, and modified locally.

The setup instructions have been updated accordingly. First-time users are asked to focus on the contents of the .env file and not interfere with the main Docker Compose file. Additional instructions can be provided in the env file to give more help about each env var.