Add support for OS X environments

[axsaucedo]

When building the environment tarball, conda-pack will include native libraries which may not work in the Docker environment. For example, if the user's environment runs in OS X, the packed env may not work in container, running Linux.

As a potential solution, we should explore leveraging Conda's CONDA_SUBDIR=linux-64 env var, which forces Conda to also install Linux binaries.