build_dockerfile: Dockerfile detection depends on filename instead of actual syntax (TODO)

[Syedowais312]

Realated to https://github.com/unikraft/kraftkit

While reviewing build_dockerfile, I noticed a TODO-related limitation where Dockerfile detection relies on a filename heuristic instead of validating the file using Dockerfile syntax.

Docker does not require a file to be named Dockerfile.
Any file can be used as a Dockerfile via:

docker build -f <file> .

So KraftKit behavior diverges from Docker/BuildKit behavior.

Existing capability

The project already includes a proper syntax validator:

github.com/moby/buildkit/frontend/dockerfile/parser

Meaning we can reliably detect Dockerfiles using grammar instead of filename heuristics.

Expected behavior

A file should be considered a Dockerfile if it successfully parses as one, regardless of filename.

Suggested direction

Replace filename detection with parser validation:

1. Attempt parsing
2. If AST exists --> treat as Dockerfile
3. Otherwise --> not buildable

This aligns KraftKit behavior with Docker and avoids false negatives in CI pipelines and multi-environment setups.

Impact

• Improves compatibility with real Docker workflows
• Removes unnecessary policy restriction
• Uses already-available canonical parser

---

Question

Was the filename restriction intentional (policy/security), or just a temporary safeguard before syntax validation was added?

[Syedowais312]

Does this direction look reasonable? If so, I can open a PR.

cc: @nderjung @craciunoiuc