chore: Migrate Skaffold presubmits to new Kokoro instance

[menahyouyeah]

Description
This PR updates the Skaffold build and test infrastructure to run presubmit checks on the new dedicated Kokoro instance (skaffold-presubmit) and GCP project (skaffold-ci-cd).

Key changes:

• Updates Makefile to conditionally use Artifact Registry in skaffold-ci-cd for presubmits, while retaining GCR for CI/Release in k8s-skaffold.

CI and Release processes remain on the existing infrastructure for now.

[gemini-code-assist]

Summary of Changes

Hello @menahyouyeah, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request orchestrates a significant infrastructure update for Skaffold's presubmit checks. The core objective is to transition these checks to a new, dedicated Kokoro environment and GCP project, leveraging Google Artifact Registry for image storage. This change enhances the build and test pipeline's scalability and organization by separating presubmit workflows from the main CI/Release infrastructure, ensuring that development iterations benefit from the new setup without disrupting existing production pipelines.

Highlights

• Migration to New Kokoro Instance: Skaffold presubmit checks are now configured to run on a new, dedicated Kokoro instance (skaffold-presubmit) and a new GCP project (skaffold-ci-cd).
• Conditional Artifact Registry Usage: The Makefile has been updated to conditionally use Google Artifact Registry (AR) for presubmit builds within the skaffold-ci-cd project, while retaining Google Container Registry (GCR) for existing CI/Release processes in the k8s-skaffold project.
• Dynamic Image Repository Configuration: Introduced dynamic setting of IMAGE_REPO_BASE, GCLOUD_AUTH_CONFIG, and GKE_LOCATION_FLAG in the Makefile, allowing the build system to adapt to either GCR or Artifact Registry based on the active GCP project.
• Updated Docker Commands and Environment Variables: All relevant docker build and docker push commands, as well as integration test invocations, have been modified to leverage the new IMAGE_REPO_BASE variable and pass necessary environment variables (GCP_PROJECT, AR_REGION, GKE_REGION, GKE_ZONE) for proper configuration.
• Presubmit Script Update: The hack/kokoro/presubmit.sh script now explicitly sets DOCKER_NAMESPACE to the Artifact Registry path for presubmit builds, ensuring images are pushed to the correct new registry.

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[gemini-code-assist]

Code Review

This pull request migrates the Skaffold presubmits to a new Kokoro instance and GCP project, introducing conditional logic in the Makefile to switch between GCR and Artifact Registry based on the GCP_PROJECT variable. The changes are logical and follow the PR's goal.

However, I've found a few critical issues that would cause the build to fail or behave incorrectly:

• The Makefile has a logic error where a variable (DEPS_DIGEST) is used without being defined in the skaffold-builder-ci target.
• Another docker build command is broken due to a missing line continuation character and inconsistent indentation.
• The presubmit script (hack/kokoro/presubmit.sh) is missing required environment variables to trigger the new build logic in the Makefile.

I've also included a high-severity suggestion to make the Makefile more robust by adding checks for required variables, and a medium-severity comment for a typo. Please address these issues to ensure the new presubmit pipeline works as expected.

[gemini-code-assist]

This line has two issues:

1. It uses spaces for indentation, while the surrounding lines and Makefile conventions use tabs. This is inconsistent.
2. It is missing a trailing \ to continue the docker build command on the next line. This will cause the build to fail.

Please fix the indentation and add the line continuation character.

		-t $(IMAGE_REPO_BASE)/skaffold:latest \

[gemini-code-assist]

The skaffold-builder-ci target uses the DEPS_DIGEST variable in a --cache-from flag, but DEPS_DIGEST is not defined within this target's scope. This will result in DEPS_DIGEST being empty, causing the cache to be missed or pointing to an incorrect image tag. You should define DEPS_DIGEST within the skaffold-builder-ci target.

skaffold-builder-ci:
	$(eval DEPS_DIGEST := $(shell ./hack/skaffold-deps-sha1.sh))

[gemini-code-assist]

This make command is missing essential environment variables (GCP_PROJECT, AR_REGION, GKE_REGION) required to trigger the new presubmit logic in the Makefile.

• GCP_PROJECT will default to k8s-skaffold, and the skaffold-ci-cd specific logic will not be used.
• AR_REGION and GKE_REGION will be unset, causing errors or incorrect behavior in the Makefile.

Please provide these variables to the make command to ensure the new presubmit infrastructure is used.

Suggested change

	GCP_ONLY=true make integration-in-docker
	GCP_ONLY=true GCP_PROJECT=skaffold-ci-cd AR_REGION=us-central1 GKE_REGION=us-central1 make integration-in-docker

[gemini-code-assist]

The new logic for GCP_PROJECT=skaffold-ci-cd relies on AR_REGION and GKE_REGION variables. However, these variables are not defined with default values, nor are there any checks to ensure they are set. If they are not provided in the environment, they will be empty, leading to malformed values for IMAGE_REPO_BASE and GKE_LOCATION_FLAG, which can cause silent failures or hard-to-debug issues. It's safer to add checks to ensure these required variables are set.

ifeq ($(GCP_PROJECT),skaffold-ci-cd)
  ifndef AR_REGION
    $(error AR_REGION must be set when GCP_PROJECT is skaffold-ci-cd)
  endif
  ifndef GKE_REGION
    $(error GKE_REGION must be set when GCP_PROJECT is skaffold-ci-cd)
  endif
  # Presubmit environment: skaffold-ci-cd project with Artifact Registry

[gemini-code-assist]

There is a typo in the comment. "hanges" should be "Changes".

Suggested change

	# hanges the current directory to where Kokoro has checked out the GitHub repository.
	# Changes the current directory to where Kokoro has checked out the GitHub repository.