[Request]: Support custom BuildKit frontends

[zachspar]

Feature or enhancement request details

Background

BuildKit is frontend-agnostic and lets you point at any LLB-speaking frontend via the # syntax=... directive (or by configuring the frontend explicitly). A good example is the frontend I've built called Yamlfile.

Current Behavior

Right now the shim has Dockerfile-specific logic that gets in the way. This results in container build effectively being locked to Dockerfile even though the underlying BuildKit instance could run other frontends without issue.

If I try to build something with a custom frontend (Yamlfile for example), I receive the following error:

Error: unknown: "dockerfile parse error on line 2: unknown instruction: apiVersion:"

Extended Behavior

It would be useful if the shim stayed out of the way on the build-definition side and just did the protocol translation + file serving generically.

This would let people use any BuildKit frontend, like Yamlfile container build -f Yamlfile:

# syntax=ghcr.io/builderhub/yamlfile:latest
apiVersion: v1alpha1
targets:
  hello:
    from: alpine
    steps:
      - run:
          command: echo "hello from Yamlfile" > /msg.txt

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