ci: fix build-docker-auto re-building Dockerfiles on every PR push

[Copilot]

GitHub's pull_request paths filter matches the full PR diff (base→head), not just the latest commit. Combined with knu/changed-files@v1 using the same full-diff comparison, once any Dockerfile appeared anywhere in a PR, build-docker would re-run for all of them on every subsequent push — even commits that only touched docs or CI files.

Changes

• Replace knu/changed-files@v1 with git diff HEAD~1 HEAD — scopes Dockerfile detection to the single commit that was just pushed instead of the cumulative PR diff; add fetch-depth: 2 to make the parent commit available
• Guard build-docker with an if condition — skips the job entirely when the matrix is empty, so the workflow starting (due to the paths filter) no longer causes spurious builds

- id: changed-files
  name: Check changed files
  run: |
    files=$(git diff --name-only HEAD~1 HEAD | grep -E '(^|/)Dockerfile\.' || true)
    ...

build-docker:
  needs: [get-files]
  if: needs.get-files.outputs.json != '[]'

[kelly-sovacool]

fixes CCBR/actions#151