feat(docker): overlay mooncake structured object store module pending release

[guapisolo]

Summary

Ship the mooncake structured object store API in the CUDA image for the mooncake rollout backend.

Motivation

miles' mooncake rollout-data object store backend needs the structured
object store API (FieldSchema, export_ref/import_ref, unified put/get,
release_result) that is merged upstream in kvcache-ai/Mooncake #2907,
#3013 and #3023 but missing from every released wheel: the v0.3.12
release branch was cut before those merges landed.

Usage

docker build -f docker/Dockerfile .
# inside the built image, the API the mooncake object-store backend imports resolves:
python3 -c "from mooncake.structured_object_store import FieldSchema, MooncakeBundleTransfer, export_ref, import_ref"

This unblocks --object-store-backend mooncake (PR #591) on images built from this Dockerfile.

Design Notes

• Key choices: overlay the single pure-Python module structured_object_store.py at pinned upstream commit 4dbe5a4c over the installed package.
• The upstream API commits touch only that module.
• The module runs unchanged on the C++ core the sglang base image already ships.
• The new RUN layer import-checks every required symbol, so an incomplete overlay fails the image build.
• Alternatives considered: bumping to the v0.3.12 wheel was rejected because its release branch predates the API merges.
• Rebuilding mooncake from source was rejected as too heavy for a temporary gap that one release will close.
• docker/Dockerfile.rocm is unchanged: whether its base image ships mooncake is unverified.

Verification

• Tests added: none; the RUN layer's import check gates every image build.
• Local round-trip with the overlaid module passes against the exact wheel the current image ships (mooncake-transfer-engine-cuda13 0.3.11.post1).
• The round-trip covers put with field schemas, ref export/import, get, release_result, cleanup_dataproto over tcp against a local mooncake_master.

Review Focus

• Scrutinize the MOONCAKE_SOS_COMMIT pin: it must cover every symbol miles/utils/object_store.py imports when PR [feat]support rollout data transfer with mooncake #591 lands.
• Scrutinize the import-check line in the new RUN layer; it is the only gate keeping an incomplete overlay out of runtime.
• Scrutinize the drop condition: the layer comment binds removal to the first mooncake release containing the API.

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.