title: Angular CDK Overlay, change default overlay container
body: <p>Is there a way to change the OverlayContainer?</p>
  
  <p>I have created a tooltip component, but sometimes I want to attach the overlay to a specific element (by default the overlay is attached to the document body).</p>
  
  <p>Here is how I am creating the overlay:</p>
  
  <pre><code>  private initOverlay(): void {
      const positionStrategy = this.overlayPositionBuilder
        .flexibleConnectedTo(this.elementRef)
        .withPositions([this.resolvedConfig]);
  
      this.overlayRef = this.overlay.create({positionStrategy});
    }
  </code></pre>
  
  <p>And this is how I am attaching a template to it:</p>
  
  <pre><code>  show(): void {
      this.overlayRef.attach(new TemplatePortal(this.tpl, this.viewContainerRef));
    }
  </code></pre>