unikube.yaml to support image tag

[Schille]

Currently it is not possible to address an arbitrary image to be run during a switch operation, but only one build locally. In order to speed up the process of the switch operation the unikube.yaml should support an image attribute.

Currently it its:

apps:
  [...]
    build:
      context: <Path to Docker build root>
      dockerfile: <Path to Dockerfile>
      target: <Dockerfile build target>

In the future the Unikube CLI should support:

apps:
  [...]
    image: my-service-image:latest

Since development images are usually already built somewhere this could be a good alternative. Instead of building the docker image on the local machine, this would pull an image from somewhere and make this the basis for the switch operation.