Test: Dev container image metadata

[chrmarti]

Refs: devcontainers/spec#95

• macOS @alexdima
• linux @lramos15
• windows @rzhao271

Complexity: 4

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• Install the devcontainer CLI 0.16.0 or later: npm i -g @devcontainers/cli.
• Have a base devcontainer.json in a base folder: base/.devcontainer.json

{
	"image": "ubuntu:latest",
	"postAttachCommand": "echo 'Hello from the postAttachCommand in the base image!'",
	"features": {
		"ghcr.io/devcontainers/features/terraform": "latest"
	},
	"customizations": {
		"vscode": {
			"extensions": ["chrmarti.regex"]
		}
	}
}

• Have a workspace devcontainer.json in a workspace folder: workspace/.devcontainer.json

{
	"image": "test-base-image",
	"postAttachCommand": "echo 'Hello from the postAttachCommand in the workspace image!'",
	"customizations": {
		"vscode": {
			"extensions": ["ms-vscode.hexeditor"]
		}
	}
}

• Build and tag the base image: devcontainer build --workspace-folder base --image-name test-base-image.
• Verify the base devcontainer.json is recorded in the base image: docker inspect test-base-image | grep metadata
• Start a workspace dev container: devcontainer up --workspace-folder workspace --remove-existing-container.
• Verify that the two postAttachCommands are run.
• Use VS Code with Dev Containers 0.254.0-pre-release or later to open the dev container.
• Verify that VS Code installs the two extensions.

Check https://github.com/devcontainers/spec/blob/chrmarti/imagemetadata/proposals/image-metadata.md for the list of properties that are carried forward on images and how they are merged. Test a few more properties, including some of the additional properties on the devcontainer.json.