chore: improve conftest maintenance path

[otjdiepluong]

Summary:

• Add unit tests in parser/docker/docker_test.go covering edge cases in the Dockerfile parser: multi-stage builds with AS aliases, comments interleaved with instructions, and continuation lines with backslashes. The existing parser is small (<200 lines) and current tests cover only the happy path; new table-driven cases would harden it without behavior changes.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.

[jalseth]

This feels like auto-generated noise (similar to ipfs/kubo#11337 (comment)).

You also have not signed the DCO. If you sign the DCO and clean up the PR, we can reconsider.