Install conda for only the docs user

[agjohnson]

Instead of installing globally, perhaps we should be installing per-user. Is this not a prescribed pattern for conda?

We should be doing something similar to this after we switch the user to docs:
https://github.com/rtfd/readthedocs-docker-images/blob/master/Dockerfile#L51-L53

Testing

Layer a separate install of conda over the existing image (to reduce time spent rebuilding the entire image) and see if this resolves problems with building projects that rely on conda.

[agjohnson]

Closing this, think we're good here.