Docker COPY doesn't work properly for symlinks using Fig

[petercv]

See this Docker issue:
moby/moby#8801

Might be a Fig issue, because plain docker doesn't seem to have this issue.

A test-case can be found here:
https://github.com/felixdv/docker-fig-symlink-testcase

[petercv]

I think I found the origin of the bug. I've traced it back to the docker/docker-py repository. The docker-py client is used by fig to build the various images. The docker-py client creates a tar archive of the build directory which is then uploaded to docker using the REST API. My suspicion is that the tar method doesn't properly handle symlinks like the normal docker client does (see https://github.com/docker/docker-py/blob/master/docker/utils/utils.py). I'm not a Python developer, but I'm gonna try fit it anyway.

[petercv]

There was indeed a bug in docker-py, however it was already fixed in version 0.6.0. So I've created a pull request that updates the requirements to include docker-py 0.6.0.