Use VIRTUAL_ENV variable for python binary

[elfosardo]

Use the VIRTUAL_ENV environment variable inside the Dockerfile to define the PYTHON variable
if VIRTUAL_ENV changes it will be automatically reflected in the PYTHON binary

[pablogsal]

Thanks! Per our contribution licensing documentation, would you mind adding a Signed-Off-By: tag to your commit message to indicate that you have read and agree to our Developer's Certificate of Origin?

That should be as easy as doing:

$ git commit --amend -C HEAD -s
$ git push -f

from a checkout of this branch.

[pablogsal]

Thanks a lot @elfosardo for your contribution 🎉

[godlygeek]

LGTM