Support dobi's enhanced caching for images built from remote TAR files and Git repos.

[aidan-mundy]

Dobi should support enhanced caching with remote build contexts.

For TAR/solo files, the implementation would most likely rely on inspection of the tarfile's contents.
For Git repos, simple caching can be done by detecting commit sha changes, with more advanced caching by detecting changes to specific files in commits that contain the cached commit in their history.

Relates to #224