21367/21230: Set up dev-jobs with virtualenv

21367/21230: Set up dev-jobs with virtualenv

The functional goal here is to do all pip installs inside the Dockerfile
inside a virtualenv to avoid the global pip install issues.

Changes that fall out of that:

* Installing python3-venv is all we need to go back to the -slim Debian
  image.
* Stop installing other Python packages we'll just install inside the
  virtualenv anyway.
* Rather than having separate arguments and code blocks for every
  package we might want to install, just set up a dedicated build
  context for the Docker image with all the packages we want to install,
  and install them all unconditionally during the image build. This is
  much less code and hopefully easier to follow.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>