Justin/custom base image support

[joostinyi]

• Adds a new top level config base_image wherein a user can supply a base image for base.Dockerfile
• Implements fail fast mechanisms checking for the os and the python version
• Introduces cache argument to the image builder to invalidate cache of RUN instructions across subsequent builds. See end of section
• Fixes base image generation issue from original PR custom base image support #268
• Tested the refactored base image generation manually