Drawing shapes and images in libgdx dialog

I am currently working on a dialog window for my android game using libgdx. This dialog window contains a collection of labels and buttons, but should also contain an image. The image represents the “remaining health indicator”, i.e. an empty indicator with a symbol indicating the health of the player. This indicator has to be filled with a colored rectangle representing the remaining amount of health (see screenshot below).

In order to render this on a dialog of libgdx, I have to draw an image and a colored rectangle (the red rectangle indicating the real amount of remaining health). I know that the dialog supports the rendering of images, but I don’t know how to first draw the rectangle.

This is the code I have so far:

public FuelFacilityDialog(GameWorld world, GuiComponent gui) { super("Health check", gui.defaultSkin);

    this.world = world;
    this.gui = gui;

    setModal(true);
    setMovable(false);
    setResizable(false);

    Image healthIndicator = new Image();
    Button button1   = new TextButton("heal", gui.defaultSkin);
    Button button4   = new TextButton("Exit", gui.defaultSkin);

    healthIndicator.setDrawable(new TextureRegionDrawable(AssetLoader.healthIndicatorV));
    healthIndicator.setScaling(Scaling.fit);
    setObject(button1, true);
    setObject(button4, false);

    Table imageTable = new Table();
    Table buttonTable = new Table();

    imageTable.add(healthIndicator).width(100).height(200);
    buttonTable.add(button1).width(100).height(50).expandY();

    this.getContentTable().padTop(20);
    this.getContentTable().padBottom(20);
    this.getContentTable().add(imageTable);
    this.getContentTable().add(buttonTable).height(200);
    getButtonTable().add(button2);
}