Add support for `COMPOSE_FILE` environment variable

[Laski]

Is your feature request related to a problem? Please describe.
In order to specify different docker-compose.yml files, it would be nice that blimp supports the environment variables that docker-compose uses, specifically the COMPOSE_FILE environment variable.

Describe the solution you'd like
When the COMPOSE_FILE environment variable is set, blimp uses it to find the Compose file(s).
This probably also implies adding support for multiple Compose files, because COMPOSE_FILE may specify many files concatenated with :.

Additional context
Probably adding CLI support would be nice too (docker-compose -f file.yaml).

[kklin]

This should be pretty straightforward to add, if you're down to take a crack at it.

There's this common function that's used to get the list of compose files to load. So you could just add a check in that function to lookup the value of COMPOSE_FILE.

Probably adding CLI support would be nice too (docker-compose -f file.yaml).

Does the -f flag for blimp already cover that? Or are you imagining a different use case?