Move .dockerignore to the repo root so it is actually read

[ronenkat]

Purpose

Move docker/.dockerignore to the repo root so it is actually read.

Both the CI build (.buildkite/cuda/scripts/build-ci-image.sh) and any build
from the repo root use the repo root as the build context and pass the
Dockerfile with --file docker/Dockerfile.ci. Builders read only
.dockerignore at the context root (or the BuildKit-only
<dockerfile>.dockerignore), so docker/.dockerignore has never had any
effect and none of its patterns are enforced.

Issue

• Related issue(s): [Bug]: docker/.dockerignore is not observed when building the image #313
• Closing keyword, only if fully resolved: Closes [Bug]: docker/.dockerignore is not observed when building the image #313

Scope

• In scope: relocate the ignore file to the build context root.
• Out of scope: changes to the patterns themselves, and docker/Dockerfile.ci.

Implementation Notes

No impact on code. The context root is the only
location honored by BuildKit, the classic builder and buildah alike, so this
fixes CI and repo-root builds with one change.

Test Plan

Build from the repo root with the CI invocation shape
(--file docker/<dockerfile> .) before and after the move, and inspect the
built image rather than trusting the ignore file.

Test Result

Identical commands; only the ignore file's location differed:

Build	ignore file location	.git	.DS_Store
before	docker/.dockerignore	PRESENT	PRESENT
control (--ignorefile docker/.dockerignore)	read explicitly	ABSENT	ABSENT
after	.dockerignore (repo root)	ABSENT	ABSENT

The control build points --ignorefile at the same file and the patterns take
effect, confirming the contents were always valid and only the location was
wrong.

Independently, the shipped CI image
public.ecr.aws/q9t5s3a7/vllm-ci-test-repo:22afe58… (source layer
sha256:de75870e…) contains opt/afd-plugin/.git, confirming the same defect
in CI.

Docs Impact

• Files updated: none
• If none, reason: no user-facing behavior change.

[jiangkuaixue123]

Thx！