Fix make docker-test by removing python 3.6. Improve docker image build.

[garyvdm]

This fixes the travis ci builds.

python 3.6 is no longer supported[1], and is not longer supported by get-pip.py [2]

The docker image build is improved by exiting on error inside shell for loop.

[1] https://peps.python.org/pep-0494/ [2] https://pip.pypa.io/en/stable/installation/#compatibility