feat!: replace bitnami/kubectl with alpine/k8s

BREAKING CHANGE: you MUST set kubectl-image to match your cluster version
    
docker.io/alpine/k8s does not provide a latest tag

Presentation

Fixes #51 (closed)

Checklist

• Documented:
  • README.md reflects any job, variable or whichever visible change
  • kicker.json reflects any job, variable or whichever visible change