Add Trivy gh action to our workflow

[jkremser]

Add the image container security scans. This GitHub action should do the trick: https://github.com/aquasecurity/trivy-action Once we release and the image got propagated to artifacthub.io, the results are then available on https://artifacthub.io/packages/helm/k8gb/k8gb

[ytsarev]

@jkremser looks like dup of #621