Pin BuildKit < v0.32 to avoid GitLab registry attestation rejection

This MR is fixing an issue that we have been seeing on all of our projects that are producing images.

For reference, the failure is:

ERROR: failed to build: failed to solve: failed to push registry.gitlab.com/sylva-projects/sylva-elements/container-images/kube-job:v1.7.3: unknown: blob unknown to registry - sha256:2d6e9212fe1a73501a912987b7abfe8b9cbbd42011b212ed59f8668532e620a3

https://gitlab.com/sylva-projects/sylva-elements/container-images/kube-job/-/jobs/16070526127

But it is the same on all other projects. This is preventing tagging new releases too. After some retries, the job succeeds, but it needs manual intervention and a lot of time.

While looking for it, I got into gitlab-org/container-registry#2367 (closed).

Reading the comments, I found gitlab-org/container-registry#2367 (comment 3683028767) which seems to be the easier and harmless solution until we get a fix. There might be other solutions too, so feel free to start a discussion.

This MR is implementing the previous suggestion.

Tested in sylva-projects/sylva-elements/container-images/kube-job!276 (closed), where the first 2 pipelines are pointing to this branch. The last pipeline is using the 1.1.3 tag to confirm that the failure is still present.