Fix docker builds by pinning PIP version

[jonathangreen]

Looks like PIP 21.0 is out and it drops Python 2 support which breaks our docker builds:
pypa/pip#6148

This should pin PIP in the docker install by pinning pip to a version that works with Python 2.

This should fix the docker containers that didn't build on this merge: a22e205

[tdilauro]

Don't want to assume that you're done, but this looks like it should do the trick.

[jonathangreen]

Thanks @tdilauro! Yes, should be good to go from my perspective.

[leonardr]

Thanks for taking care of this.