Fix cloudbuild to include an extra tag

[maxcao13]

Followup to #54

This commit improves the Makefile to allow more configurable build options. Also includes an EXTRA_TAG variable so automatic builds push images with branch name tags.

Related to kubernetes/test-infra#35802 because I realized we probably want to automatically push stage images based on our release branches.

Took inspiration from this Makefile: https://github.com/kubernetes-sigs/lws/blob/main/Makefile

Tested with my own repository and injected credentials:

$ gcloud builds submit --config=cloudbuild.yaml
...omitted
#17 pushing quay.io/macao/my-karpenter-test:main with docker
#17 pushing layer 8fa10c0194df 0.7s done
...omitted...
#17 DONE 0.7s

#5 [stage-1 1/3] FROM gcr.io/distroless/static:nonroot-amd64@sha256:127eb3c971acb50a3c7b511a978bb497b56add4c91225e57605b3b43cc0a9662
PUSH
DONE
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ID                                    CREATE_TIME                DURATION  SOURCE                                                                                        IMAGES  STATUS
d4885e4a-0c40-468c-a142-d676a88b3191  2025-10-29T20:41:50+00:00  5M40S     gs://default-476521_cloudbuild/source/1761770498.694338-c5c121acf11b415ca19d198fb59c3088.tgz  -       SUCCESS

[maxcao13]

@ttsuuubasa Idk if anyone is relying on the old existing Makefile variables, but since this repo is in alpha stage I think we can make changes like this, but just giving a heads up.

/cc @elmiko

[elmiko]

/approve
/lgtm

/hold

this looks fine to me, feel free to unhold if you are ok @maxcao13

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: elmiko, maxcao13

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[maxcao13]

/unhold