Change in environment variable setting behaviour in 3.6 image?

I'm not entirely sure if this issue is here, docker, docker-compose or alpine itself. I'm starting here and I'm hoping you'll be able to tell me if it's elsewhere. :)

Something seems to have changed with environment variable setting behaviour between alpine 3.5 and 3.6's docker images when using it via docker-compose.

I've created the simplest project I can to reproduce this: https://github.com/liamjones/alpine-docker-environment-issue. This project creates containers from the 3.5 and 3.6 images and runs the env command.

If you `docker-compose up --build alpine-35` you'll see that `foo.bar` and `FOO_BAR` are set in the environment.

If you `docker-compose up --build alpine-36` you'll see that `foo.bar` is not set in the environment but `FOO_BAR` still is.

Interestingly, if the Dockerfile's `CMD` is overridden in `docker-compose.yml` (with the exact same command) then `foo.bar` starts working again for 3.6. See `docker-compose up --build alpine-36-overriddencommand` for a demonstration of this.