Use env variables to denote volume paths

[dr-orlovsky]

It’s possible to use environment variables in your shell to populate values inside a Compose file

• define default .env file within compose directory using volumes
• define custom macos.env, windows.env & linux.env files using default OS-specific paths for volumes
• use ENV variables in compose file volume dfinitions

[dr-orlovsky]

@zoedberg feel free to grab this task if you'd like to

[dr-orlovsky]

Not required b/c of #6 and https://github.com/LNP-BP/docker/blob/master/external.env