Substitution with service environment

[lisongmin]

This pr add the ability to substitute the variable with the service's environment.
This allows the use the service's environment within the service block itself.

For instance, in the provided example:

services:
  app:
    image: "abc:${version}"
    build:
      context: .
      args:
        VERSION: "${version}"
    environment:
      version: v1.0

The version environment is defined within the app service and
it is then used to define the image and VERSION build argument
for the same service.

[p12tic]

Looks great, thanks. I had one comment, but aside from that the PR can go in.

[p12tic]

Rebased.