image: Support remote build and config contexts

The Blubber (>= v1.0.0) and Dockerfile frontends both support building
from remote contexts such as Git URLs. Using remote contexts is an
important part of generating complete software provenance metadata and
a step toward more repeatable builds.

Let's support building from remote build contexts in Kokkuri as well.

Note that either the BUILD_FRONTEND environment variable or
--frontend CLI flag are now required when specifying a remote config
context since we can't reliably parse the syntax line from a remote
config.

---

Related MRs:

• !102 (merged)
• !101 (merged) (This MR)
• main