Restrict reproducible-build dirty check to dockerignore-allowed paths

[potiuk]

Summary

Restrict the reproducible-build dirty check in
airflow-core/hatch_build.py to paths inside the docker build context
(per .dockerignore). Without this, official airflow-core releases
ship with a misleading git_version = .dev0+<SHA>.dirty even when
built from a clean tag, because the deny-by-default .dockerignore
excludes ~220 tracked top-level files from COPY . /opt/airflow,
so git is_dirty() inside the container reports them as deletions.

Verified against 3.2.2rc3:

• before fix: apache_airflow_core wheel's airflow/git_version =
  .dev0+cde4885818be51a6cdcfdf9275e100bf070025de.dirty
• after fix: .release:<SHA>

Test plan

• Built apache-airflow-core wheel via docker on main with the fix —
  git_version is now .release:<SHA> (no .dirty suffix)
• Dirty detection still fires for genuine in-context changes:
  modifying airflow-core/README.md → restricted check returns True
• Dockerignored-only changes no longer trip dirty:
  modifying .asf.yaml → restricted check returns False
• prek run mypy-airflow-core passes
• ruff format / ruff check clean
• Falls back to repo.is_dirty() when .dockerignore is missing
  or has no ! entries (preserves prior behavior in unusual contexts)

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Opus 4.7)

Generated-by: Claude Code (Opus 4.7) following the guidelines

[potiuk]

@jscheffl You're right — this is build-tooling, not user-facing. I've dropped the newsfragment from this PR.

Rather than just fixing it here, I'd prefer to update the expectation for everyone so agents stop adding these in the first place: opened #67982, which adds a golden rule to AGENTS.md (the file all agents read via the CLAUDE.md symlink) — never create a newsfragment unless the change is certainly user-facing, and when in doubt omit it.

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Drafted-by: Claude Code (Opus 4.8); reviewed by @potiuk before posting

[jscheffl]

@jscheffl You're right — this is build-tooling, not user-facing. I've dropped the newsfragment from this PR.

Rather than just fixing it here, I'd prefer to update the expectation for everyone so agents stop adding these in the first place: opened #67982, which adds a golden rule to AGENTS.md (the file all agents read via the CLAUDE.md symlink) — never create a newsfragment unless the change is certainly user-facing, and when in doubt omit it.

Drafted-by: Claude Code (Opus 4.8); reviewed by @potiuk before posting

I thought the statement was clear before... but let's hoep the PR makes it clearrer.