fix: always write devcontainer.metadata label as JSON array (#1199)

* fix: always write devcontainer.metadata label as JSON array

When there is only one metadata entry (e.g. docker-compose devcontainer
with no features), `getDevcontainerMetadataLabel` wrote a bare JSON
object instead of an array. This violates the spec which states the
label "can contain an array of json snippets" and causes tools like Zed
that expect an array to fail when attaching to an existing container.

Always wrap the metadata in an array regardless of the number of entries.

Spec reference: https://containers.dev/implementors/json_reference/
Fixes #1054

* cleanup: remove unnecessary spaces

* test: add test for single metadata entry always being an array

Author: fechu

src/spec-node/imageMetadata.ts

@@ -497,11 +497,9 @@ export function getDevcontainerMetadataLabel(devContainerMetadata: SubstitutedCo
 	if (!metadata.length) {
 		return '';
 	}
-	const imageMetadataLabelValue = metadata.length !== 1
-		? `[${metadata
-			.map(feature => ` \\\n${toLabelString(feature)}`)
-			.join(',')} \\\n]`
-		: toLabelString(metadata[0]);
+	const imageMetadataLabelValue = `[${metadata
+		.map(feature => ` \\\n${toLabelString(feature)}`)
+		.join(',')} \\\n]`;
 	return `LABEL ${imageMetadataLabel}="${imageMetadataLabelValue}"`;
 }
 

src/test/imageMetadata.test.ts

@@ -432,6 +432,22 @@ describe('Image Metadata', function () {
 			assert.strictEqual(label.replace(/ \\\n/g, ''), `LABEL devcontainer.metadata="${JSON.stringify(expected).replace(/"/g, '\\"')}"`);
 		});
 
+		it('should create array label for single metadata entry (docker-compose with Dockerfile, no features)', () => {
+			// When there is only one metadata entry, the label should still be a JSON array.
+			// Regression test for https://github.com/devcontainers/cli/issues/1054
+			const label = getDevcontainerMetadataLabel(configWithRaw([
+				{
+					remoteUser: 'testUser',
+				}
+			]));
+			const expected = [
+				{
+					remoteUser: 'testUser',
+				}
+			];
+			assert.strictEqual(label.replace(/ \\\n/g, ''), `LABEL devcontainer.metadata="${JSON.stringify(expected).replace(/"/g, '\\"')}"`);
+		});
+
 		it('should merge metadata from devcontainer.json and features', () => {
 			const merged = mergeConfiguration({
 				configFilePath: URI.parse('file:///devcontainer.json'),