chore(testing-tools): Pull grpcurl from our own mirror

[dervoeti]

Description

testing-tools was the only image left in this repo that pulled a base image directly from Docker Hub, by mutable tag: FROM fullstorydev/grpcurl:v${GRPCURL_VERSION}.

I think we should mirror that image, because:

• We pull whatever fullstorydev currently has behind v1.9.3. A compromised upstream account or a re-pushed tag lands in our testing-tools image without anything on our side noticing.
• If the upstream repository or tag goes away, or Docker Hub rate-limits or blocks us, the build breaks and old versions become unreproducible.

Not super important since this is just testing-tools, but no huge effort either. Other external images we consume already go through our Harbor mirror (like git-sync or astral-sh/uv).

I already mirrored the image using the workflow: https://github.com/stackabletech/docker-images/actions/runs/35729393524

Definition of Done Checklist

Note

Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant.

Please make sure all these things are done and tick the boxes

• Changes are OpenShift compatible
• All added packages (via microdnf or otherwise) have a comment on why they are added
• Things not downloaded from Red Hat repositories should be mirrored in the Stackable repository and downloaded from there
• All packages should have (if available) signatures/hashes verified
• Add an entry to the CHANGELOG.md file
• Integration tests ran successfully

TIP: Running integration tests with a new product image

The image can be built and uploaded to the kind cluster with the following commands:

boil build <IMAGE> --image-version <RELEASE_VERSION> --strip-architecture --load
kind load docker-image <MANIFEST_URI> --name=<name-of-your-test-cluster>

See the output of boil to retrieve the image manifest URI for <MANIFEST_URI>.