use an env variable for the docker-compose file

Hi,
I think we should have a way to use a .env, a config file in /etc or a variable from the environment to look at the docker-compose file. Perhaps could add the support for multiple one (find dir1 dir2 ...) ?

Start by locating the entry point where Captain selects the docker-compose file, then inspect how it handles command-line and environment configuration. Define and test the precedence between a .env file, /etc configuration, and an environment variable, including the requested support for multiple compose files; done means the selected configuration reliably starts the intended project.

Written by the indexing model from the issue text.