Fix config --variables not honoring the --format flag

When providing the --variables with --format flag, the current
implementation always printed in human readable form.
This patch correctly add the missing format in the formatter.Print
function, making the commands behave as an user would expect.

Example:
`config --variables --format json`

Signed-off-by: Alessio Perugini <[email protected]>

cmd/compose/config.go

@@ -124,12 +124,15 @@ func configCommand(p *ProjectOptions, dockerCli command.Cli) *cobra.Command {
 				return runEnvironment(ctx, dockerCli, opts, args)
 			}
 
+			if opts.Format == "" {
+				opts.Format = "yaml"
+			}
 			return runConfig(ctx, dockerCli, opts, args)
 		}),
 		ValidArgsFunction: completeServiceNames(dockerCli, p),
 	}
 	flags := cmd.Flags()
-	flags.StringVar(&opts.Format, "format", "yaml", "Format the output. Values: [yaml | json]")
+	flags.StringVar(&opts.Format, "format", "", "Format the output. Values: [yaml | json]")
 	flags.BoolVar(&opts.resolveImageDigests, "resolve-image-digests", false, "Pin image tags to digests")
 	flags.BoolVarP(&opts.quiet, "quiet", "q", false, "Only validate the configuration, don't print anything")
 	flags.BoolVar(&opts.noInterpolate, "no-interpolate", false, "Don't interpolate environment variables")
@@ -408,7 +411,16 @@ func runVariables(ctx context.Context, dockerCli command.Cli, opts configOptions
 
 	variables := template.ExtractVariables(model, template.DefaultPattern)
 
-	return formatter.Print(variables, "", dockerCli.Out(), func(w io.Writer) {
+	if opts.Format == "yaml" {
+		result, err := yaml.Marshal(variables)
+		if err != nil {
+			return err
+		}
+		fmt.Println(string(result))
+		return nil
+	}
+
+	return formatter.Print(variables, opts.Format, dockerCli.Out(), func(w io.Writer) {
 		for name, variable := range variables {
 			_, _ = fmt.Fprintf(w, "%s\t%t\t%s\t%s\n", name, variable.Required, variable.DefaultValue, variable.PresenceValue)
 		}