feat: support custom BuildKit frontends

[zachspar]

Background

This PR implements my feature request in #82 to enable building images using custom BuildKit frontends (ie. # syntax=ghcr.io/builderhub/yamlfile:latest, BUILDKIT_SYNTAX, and explicit gateway.v0 metadata), while keeping the existing native dockerfile.v1 path for plain Dockerfiles.

Previously the shim always parsed the build file as a Dockerfile before the build started and always ran a hard-coded dockerfile.v1 frontend. That broke external frontends like Yamlfile with errors such as unknown instruction: apiVersion:.

New Approach

Native Dockerfiles keep existing shim-specific behavior. External frontends delegate to the declared frontend image via BuildKit’s gateway mechanism.

Eventually it would be nice if we wire frontend / frontend-opt metadata from container, but that is not a requirement to achieve the desired behavior.

flowchart TD
    Input[Build file bytes] --> Resolve[resolveFrontend]
    Resolve -->|native| Native[Skip external path]
    Native --> Parse[parseDockerfileMetadata + dockerfile.v1]
    Resolve -->|external| Skip[Skip Dockerfile parse]
    Skip --> Stage[stageBuildDefinition + fssync staging]
    Stage --> Gateway[forwardGateway to gateway.v0]

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