2: add virtualenv

Tested with:
```
proppy@noir:~/python/2$ docker run -ti python:2 bash
root@6b01e15ce5d6:/usr/src/python# virtualenv
```

:3 already has `pyvenv`

Ahh, nevermind, just noticed your comment on the description about `pyvenv` - but the separate `RUN` line question still applies.  Do we gain anything from this being together like this?