Add support for signing images

Description

Support signing the images built with the Docker template.

Implementation ideas

Cosign seems to be widely adopted for this purpose:

• https://about.gitlab.com/blog/2024/09/04/annotate-container-images-with-build-provenance-using-cosign-in-gitlab-ci-cd/
• https://www.civo.com/learn/supply-chain-security
• https://sysdig.com/blog/secure-kubernetes-deployment-signature-verification/