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

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}"`;
 }