Preserve Dockerfile devcontainer.metadata during image build

[RossBugginsNHS]

Attribution

Submitted by Ross Buggins after interactive investigation and drafting assistance from GitHub Copilot. The changes and PR content were reviewed and submitted by the author, not posted by an unattended bot.

Summary

This PR preserves user-declared Dockerfile devcontainer.metadata during CLI image-build metadata computation so that final effective metadata composition does not drop user image metadata entries.

Related Discussion

Spec/proposal issue:

• Proposal: preserve Dockerfile devcontainer.metadata through CLI metadata composition spec#739

Fork issue used during preparation:

• Proposal: preserve user Dockerfile devcontainer.metadata in CLI build paths RossBugginsNHS/devcontainers.cli#2

Problem

Expected

If a Dockerfile includes LABEL devcontainer.metadata=..., those entries should survive CLI build composition and be merged with base image, feature, and config-derived metadata.

Actual

In affected Dockerfile build paths, especially feature-enabled ones, the final effective devcontainer.metadata can exclude metadata declared directly in the user Dockerfile.

Impact

This can silently remove runtime-relevant configuration encoded in metadata, including examples like:

• mounts
• lifecycle hooks such as postCreateCommand
• other merged configuration fields expected to persist through image extension

Root Cause

internalGetImageBuildInfoFromDockerfile(...) derived metadata from the resolved base image, but did not reliably merge metadata declared directly in Dockerfile LABEL devcontainer.metadata=... instructions into the metadata set later used for final wrapper/feature label generation.

What This PR Changes

1. Preserve Dockerfile label metadata in Dockerfile build-info extraction

File:

• src/spec-node/imageMetadata.ts

Changes:

• Parse devcontainer.metadata values from Dockerfile LABEL instructions.
• Merge parsed Dockerfile metadata with existing base-image metadata during internalGetImageBuildInfoFromDockerfile(...).

2. Reuse shared metadata JSON parsing

File:

• src/spec-node/imageMetadata.ts

Changes:

• Extract shared metadata parsing logic so image-label parsing and Dockerfile-label parsing use the same JSON handling path.

3. Add regression tests

File:

• src/test/dockerfileUtils.test.ts

Added tests:

• preserves metadata declared in Dockerfile labels
• parses metadata declared with a quoted label key

These verify that Dockerfile-declared metadata is present in both raw and substituted metadata returned by internalGetImageBuildInfoFromDockerfile(...).

Validation

Executed locally:

• Targeted tests:
  • mocha -r ts-node/register --exit src/test/dockerfileUtils.test.ts
  • Result: 71 passing
• Lint:
  • eslint src/spec-node/imageMetadata.ts src/test/dockerfileUtils.test.ts
  • Result: clean
• Type-check:
  • npm run type-check
  • Result: success

Why This Is Safe

• Scope is narrow and limited to Dockerfile metadata extraction/merge in the image-build info path.
• No public CLI contract or flag behavior changes.
• Regression coverage added around the new behavior.

Notes

This PR is intended to align implementation with the expected metadata-composition behavior. If maintainers would prefer to settle semantics first in the spec discussion, this PR can still serve as a concrete implementation reference and regression test basis.

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