Add image builder helper tools

[Victoremepunto]

Add image_builder module, provides

• build (for building and tagging images)
• tag (for tagging images)
• push (for pushing images and tags)
• get_image_tag (for calculating appropriate build image_tag depending on context)

This also adds a refactor for common and core functions to log error and debug messages, as well as a refactor of file names and variables to confort to a naming convention.

[maknop]

Solid changes, great work! LGTM

[bsquizz]

I spotted a couple minor things, mostly looks good, I think it will mostly come down to testing everything well :)

Only thing I will add is that I think each library/module should have documentation in the README (or in another .md file) that lists which env vars can be set for the library by the caller, what the defaults are if they are unset, and what each env var controls... but that can be done in a follow-up PR.