Support propagate-environment

[pecigonzalo]

It would be great to support propagate-environment as done in https://github.com/buildkite-plugins/docker-buildkite-plugin

[ianwremmel]

Was about to open a request for this. I'm currently doing this janky bash/awk thing, but it can only pass through the variables that are set on the box doing pipeline upload, it can't automatically know about env keys (I had to add NODE_ENV manually):

docker_compose_environment: &docker_compose_environment
  - AWS_DEFAULT_REGION
$(env | awk -F= '/^BUILDKITE/ {print "  - " $1}')
  - CI
  - NODE_ENV
EOT

[toote]

Duplicate of #61