fix(cli): first run blockers for build

[savme]

This PR fixes several UX issues and improves error reporting in datumctl compute build.

• buildkit is now found using the same logic the docker CLI uses
• push and build inspect read registry credentials the same way docker push does
• build inspect can read the .tar files and folders that build --output saves
• a Kraftfile-based build is only triggered when requested explicitly with --kraftfile
• building the same input twice gives the same digest
• saving to the same --output folder twice now works
• problems with --output and --push are reported before the build starts

Fixes #240