Add support for 'env-file' key

I wasn't going to make PR until adding test, but it seems it would save people some work so in the meantime, here it is.

This adds support for the `env-file` key, with a path to a file in the same format `docker run --env-file` would take. It can work together with `environment` key, meaning the variables defined in the file will be overriden by anything `environment` specifies.