Is it possible to configure a fig.yml in such way that you can pass on execution time variables instead of hard-wiring them inside the fig.yml?

I think a generic variable injection during execution would be quite useful for various use cases.

e.g:

jenkins:
  image: aespinosa/jenkins:latest
  ports:
    - "8080"
  hostname: ${HOSTNAME}

HOSTNAME=ci fig up

That could inject the variable HOSTNAME inside the fig.yml during execution and execute a docker run with hostname ci.

ps. This is different than passing environment variables inside docker which is already supported (http://www.fig.sh/yml.html#environment)