Make targets should error if `REGISTRY` is unset

[mboersma]

What steps did you take and what happened:

I naively ran make test-e2e-local and it failed with an error that took a bit of investigation to fix:

DOCKER_BUILDKIT=1 docker build --build-arg builder_image=docker.io/library/golang:1.21.5 --build-arg goproxy=https://proxy.golang.org,direct --build-arg ARCH=arm64 --build-arg ldflags="-X 'sigs.k8s.io/cluster-api-addon-provider-helm/version.buildDate=2024-03-04T18:11:00Z' -X 'sigs.k8s.io/cluster-api-addon-provider-helm/version.gitCommit=a63ed8ffebda615e93f8f4c48a362b4a94746b5d' -X 'sigs.k8s.io/cluster-api-addon-provider-helm/version.gitTreeState=clean' -X 'sigs.k8s.io/cluster-api-addon-provider-helm/version.gitMajor=0' -X 'sigs.k8s.io/cluster-api-addon-provider-helm/version.gitMinor=1' -X 'sigs.k8s.io/cluster-api-addon-provider-helm/version.gitVersion=v0.1.1-alpha.1.11-a63ed8ffebda61' -X 'sigs.k8s.io/cluster-api-addon-provider-helm/version.gitReleaseCommit=ca44da536ee11b5ffd27f6f1a75f8a9552266caf'" . -t gcr.io//cluster-api-helm-controller-arm64:dev
[+] Building 0.0s (0/0)                                                                                        docker:desktop-linux
ERROR: invalid tag "gcr.io//cluster-api-helm-controller-arm64:dev": invalid reference format

What did you expect to happen:

Perhaps the Makefile could warn (or specific targets could error) if REGISTRY isn't set. Making this clear would help contributors figure out how to run the e2e tests locally.

/kind bug

[Jont828]

/assign @mboersma

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/close not-planned

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@k8s-triage-robot: Closing this issue, marking it as "Not Planned".

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The Kubernetes project currently lacks enough active contributors to adequately respond to all issues and PRs.

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/close not-planned

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