[raycicmd] Give job containers the agent's name

[elliot-barn]

Description

Adds add-host: rayci.localhost:host-gateway to the docker buildkite plugin config that makeRayDockerPlugin emits for job containers.

Wanda's docker build (#501) and ray's ci/ray_ci/linux_container.py both already pass this mapping, so a service listening on the agent — the package index proxy — is reachable by one stable name from an image build and from a test container. Containers rayci starts through the docker buildkite plugin were the one launcher missing it.

The gap became visible when ray's CI images started baking the name into PIP_INDEX_URL (ray-project/ray#65578, armed by ray-project/ray#65605): any job_env step that runs pip at runtime resolves the baked index and fails DNS with [Errno -5] No address associated with hostname. Observed on ray's doc: check API annotations / check API doc consistency (via setuptools fetch_build_eggs for cython), pyrefly type check and build: pip-compile dependencies (direct runtime pip installs).

The mapping is emitted unconditionally, matching the other two launchers. The docker buildkite plugin v5.8.0 supports add-host natively (verified in its plugin.yml). Agents where nothing listens on the name are unaffected — the entry only matters to a container that dials it.

Testing

• New TestMakeRayDockerPlugin_addHost asserts the emitted mapping.
• go test ./raycicmd passes; go test ./... green except pre-existing local-env failures in raycilint (host git too old for --initial-branch), identical on clean main.

Rollout note: after this merges and a release is cut, ray consumes it via .rayciversion; the pyrefly type check step is a live canary since it still pip-installs at runtime.

AI assistance (Claude Code) was used to prepare this change.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request configures the Docker plugin in raycicmd/bk_pipeline.go to include the add-host mapping for rayci.localhost:host-gateway, preventing DNS resolution failures during runtime pip installations. It also adds a corresponding unit test in raycicmd/bk_pipeline_test.go to verify this configuration. There are no review comments, and I have no additional feedback to provide.