Ability to define secrets from environment variables

[ndeloof]

What is the problem you're trying to solve
secrets can be defined from a file, but in some circumstances, short-lived secrets are actually exposed by platform as environment variables (typically, GITHUB_TOKEN).
We should offer an opportunity for user to define a secret to be set from environment variable.

Describe the solution you'd like

secrets:
  github_token:
    environment: GITHUB_TOKEN

Additional context
Ability to pass build time secrets, like docker build --secret id=github_token,env=GITHUB_TOKEN does

[EricHripko]

Agreed, it's be pretty awesome to be able to inherit some of the environment variables from the host in Compose 🙌