Update docker-push target to use IMG variable

[RobbertH]

Describe your changes

We wanted to make docker-build docker-push IMG=... but it didn't take into account the IMG.
This PR fixes that.

Issue ticket number and link

Checklist before requesting a review

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests.
• I tested the changes with the following distributions:
  • Kind
  • MiniKube
  • MicroK8s
  • Rancher RKE2
  • Azure AKS
  • GCP GKE (Ubuntu nodes)
  • AWS EKS (AmazonLinux2 nodes)
  • AWS EKS (Ubuntu nodes)
  • Digital Ocean Kubernetes

[vdice]

Hi @RobbertH, thanks for the PR.

Alas, the docker-push-% target is currently used elsewhere to push images for e.g. runtime-class-manager or shim-downloader, etc, based on the $* arg provided. I do see the non-uniformity though, with the docker-build target conversely using ${IMG}.

To unblock you, would make docker-build docker-push REGISTRY=... TAG=... work for your needs? It isn't as streamlined as just using IMG but that should work today.

Otherwise, we'd just need to refactor things a bit more to align the targets on IMG.