Pass variables inside fig.yml on runtime

Problem

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: <pre> jenkins: image: aespinosa/jenkins:latest ports: - "8080" hostname: ${HOSTNAME} </pre> 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)