Update docs re: docker images

[molpopgen]

#1396 changed the docker image setup a bit.

We need to make sure that the docs note that the package is now installed into a virtual environment.

We should also document that the provided docker image is probably not the best way to use docker.
In general, it would be better for users to make their own images that:

• start from a fresh image, such as one of the Python official images
• install fwdpy11 "in the usual way", relying on a wheel.
• install their other dependencies at the same time, preferably from some kind of fully-pinned requirements file.