Allow Tox Tests to run from separate image from production built image

Currently our tox testing setup assumes that the image that has been built within the pipeline's build job will contain tox.

In order to keep our production images small and only containing what's needed to run the production app, we want to exclude tox from them.

To continue to run tox tests within the pipeline, allow the tox test job to use a separate image which includes the tox command.