(#175) Make holos render --cluster-name flag optional

Problem:
Rendering the whole platform doesn't need a cluster name.

Solution:
Make the flag optional, do not set the cue tag if it's empty.

Result:
Holos renders the platform resource and proceeds to the point where we
need to implement the iteration over platform components, passing the
platform model to each one and rendering the component.

internal/builder/builder.go

@@ -99,7 +99,9 @@ func (b *Builder) Instances(ctx context.Context) ([]*build.Instance, error) {
 	}
 
 	// Refer to https://github.com/cue-lang/cue/blob/v0.7.0/cmd/cue/cmd/common.go#L429
-	cfg.Tags = append(cfg.Tags, "cluster="+b.Cluster())
+	if b.Cluster() != "" {
+		cfg.Tags = append(cfg.Tags, "cluster="+b.Cluster())
+	}
 	log.DebugContext(ctx, fmt.Sprintf("cue: tags %v", cfg.Tags))
 
 	return load.Instances(args, &cfg), nil

internal/cli/render/render.go

@@ -28,11 +28,7 @@ func New(cfg *holos.Config) *cobra.Command {
 	cmd.Flags().AddGoFlagSet(flagSet)
 
 	cmd.RunE = func(cmd *cobra.Command, args []string) error {
-		if cfg.ClusterName() == "" {
-			return errors.Wrap(fmt.Errorf("missing cluster name"))
-		}
-
-		ctx := cmd.Context()
+		ctx := cmd.Root().Context()
 		log := logger.FromContext(ctx).With("cluster", cfg.ClusterName())
 		build := builder.New(builder.Entrypoints(args), builder.Cluster(cfg.ClusterName()))