cli: k8s: add tests for building using buildkit

[ivotron]

Add a test for the k8s runner that checks that a workflow that builds images using buildkit works. For a guide on how a workflow would use buildkit, see here.

The tests would be added in this file. These make use of KinD to test in docker (see how this is done from the travis config).

[VD44]

I will start working on this issue.

[ivotron]

thanks! please let us know if you have any questions