Add Checks for Requirements

add checks for each action (build-image, push-image, tag-source): this will check for necessary things (e.g. for build-image you need docker so make sure it is available, for push you need a repo to push to and credentials, for tag-source you need a git repo and push privs, etc.)

Currently we have two requirements:

• Docker (if build/push)
• git (if source tag)

This logic needs thought - do we run the check on every run or only if requested/needed? Perhaps we check for a .requirement-check file as a sort of cache?