Render container as konva container instead of template

[boboldehampsink]

Fixes https://github.com/konvajs/vue-konva/issues/123#issuecomment-1011857689

[lavrton]

I am still not sure what exactly this Pull Request is fixing. And I am not sure it is a good idea to use container in render.

[boboldehampsink]

@lavrton it fixes triggering of this line: https://github.com/konvajs/vue-konva/blob/master/src/utils/index.js#L38

It is triggered by these lines: https://github.com/konvajs/vue-konva/blob/master/src/components/KonvaNode.js#L110-L113

which try to render a <template> tag inside the stage, which is not a Konva node and thus forbidden. This PR attempts to use a proper Konva node here: https://konvajs.org/api/Konva.Container.html