OCI build from Dockerfile

[preminger]

Description of the Pull Request (PR):

This PR adds support for building an OCI-SIF from a Dockerfile (build --oci).

This fixes or addresses the following GitHub issues:

• Fixes Build from Dockerfile to OCI-SIF (build --oci) #2218

[dtrudg]

Just starting to look through and note a few things to think about later. Not necessarily any action needed here.

[preminger]

Some info that pertains to storage, locking, caches and dbs, etc.:

• The buildkitd.sock and corresponding buildkitd.lock file get created in /run/buildkit/ (when we run as root) or /run/user/<uid>/buildkit/ (when we run as a regular user).
  • If I understand the workings of /run correctly, local-only lock will work just fine here, since /run is always local anyway.
  • That said, I don't think we should necessarily prohibit a pair of these builds running simultaneously, each launching their own ad-hoc buildkitd & tearing it down when they're done.
  • I'll explore the possibility of adding a PID-based string to the dirnames above (/run/buildkit/ / /run/user/<uid>/buildkit/).
    • ETA: Implemented.

buildkitd's cache, however - which is the part that uses a bolt db - is saved in ${HOME}/.local/share/buildkit when running as a regular user. (/var/lib/buildkit when running as root.)

• I think it would be possible to modify the code so that it uses /var/tmp for this, if we decide that would be preferable

[dtrudg]

Had a pretty good think through what's here, and left a bunch of comments / questions. Looking good to me so far.