Sign agentk images

From https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/container-loader/-/merge_requests/3#note_2159923378:

We do this as standard across all images built in Infrastructure, using a common docker build task (https://gitlab.com/gitlab-com/gl-infra/common-ci-tasks/-/blob/main/docker.md#signing-and-verification)

This ensures that we can guarantee the provenance of all images built. Using keyless signing is fairly low-overhead, since we don't need to worry about managing any keys, but ensures that images are build in the tag pipelines in CI, which can prevent tampering.

GitLab offers a tutorial on how to add keyless signing, here: https://docs.gitlab.com/ee/ci/yaml/signing_examples.html