Support devcontainer.json with build field

[Copilot]

Plan: Support devcontainer.json with "build" field

• Add "build" field to DevcontainerConfig struct
• Update ensureDevcontainerConfig to preserve "build" field when updating existing config
• Add test for devcontainer with "build" instead of "image"
• Verify changes with existing tests
• Run make agent-finish for final validation
• Revert .devcontainer/devcontainer.json to original state

Summary

Successfully added support for devcontainer.json files that use the "build" field instead of the "image" field.

Changes Made:

1. Added DevcontainerBuild struct to represent build configuration:

   • dockerfile: Path to Dockerfile
   • context: Build context path
   • args: Build arguments

2. Updated DevcontainerConfig struct:

   • Added Build *DevcontainerBuild field with omitempty tag
   • Changed Image field to use omitempty tag to avoid empty string in JSON

3. Added comprehensive test (TestEnsureDevcontainerConfigWithBuildField):

   • Validates that existing build field is preserved when updating
   • Verifies that image field is not set when build is present
   • Confirms all other devcontainer features are merged correctly

4. Restored .devcontainer/devcontainer.json:

   • Reverted accidental modification of repository's devcontainer.json
   • Original "build" field is now preserved

How It Works:

• When reading an existing devcontainer.json, the parser now recognizes both "image" and "build" fields
• The existing update logic already preserves all fields by copying the entire config (config = *existingConfig)
• The omitempty JSON tags ensure clean serialization (no empty fields in output)
• When creating a new devcontainer.json from scratch, we still use the "image" field as before

Test Results:

All devcontainer tests pass:

• ✅ TestEnsureDevcontainerConfig
• ✅ TestEnsureDevcontainerConfigWithAdditionalRepos
• ✅ TestEnsureDevcontainerConfigWithCurrentRepo
• ✅ TestEnsureDevcontainerConfigWithOwnerValidation
• ✅ TestEnsureDevcontainerConfigUpdatesOldVersion
• ✅ TestEnsureDevcontainerConfigMergesWithExisting
• ✅ TestEnsureDevcontainerConfigWithBuildField (NEW)

Original prompt

Update the devcontainer.json update code to handle case where the "image" field is missing and the "build" field is used instead:

  "build": {
    "dockerfile": "Dockerfile"
  },

See current devcontainer.json

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