Add extends property for configuration inheritance

[Sam Frost (SamuelFrost)]

(note for reviewers: this is vibe coded based off the existing PR and issues. I have not reviewed it yet beyond minimal local testing-- hence the draft status)

Summary

Teams sharing a codebase often need slightly different devcontainer.json settings without copying the whole file. Issue #22 (labeled finalization) proposed an extends property for that.

This PR adds extends to the schema and devcontainer.json reference so one configuration can inherit another JSON or JSONC file in the same repository.

Companion Dev Container CLI implementation: devcontainers/cli#1306

Property

Property	Type	Description
extends	string	A relative path to a JSON or JSONC file in the same repository to use as a base configuration. The referenced file is merged with this file using the image metadata merge logic. Absolute paths and URLs are not supported. The extends property is not present in the merged result.

Changes

• Add extends to schemas/devContainer.base.schema.json.
• Document configuration inheritance in the devcontainer.json reference, using the examples from Addition of "extends" top level property to enable simple configuration inheritance #22.
• State that extends uses the same merge rules as image metadata (forwardPorts union, hostRequirements max, last-wins for scalars, and so on).

Technical choices

Issue #22 originally described Docker Compose–style deep merge. Review of CLI PR #311 (Christof Marti, Josh Spicer, Chuck Lantz) settled on the image metadata merge logic so inheriting a file and reusing a prebuilt image behave the same way. This spec change matches that.

Nested extends is allowed; cycles, absolute paths, and URLs are not. Cross-repository references are left as future work, as in #22.

Follow-up

CONTRIBUTING.md also asks for a PR against the devcontainer.json reference in vscode-docs once maintainers are on board.

Finalizes #22.

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