fix: image CI/CD pipeline

Replace ubuntu:jammy-podman image to avoid docker-in-docker (podman-in-docker) execution. Replace with:

• kaniko in the build-image job
  • Preffered tool for building images in GitLab.
  • See https://docs.gitlab.com/ee/ci/docker/using_kaniko.html.
• skopeo in the release-image job
  • kaniko is not suitable for image re-tagging.
  • skopeo is specialized on image manipulation.
  • See https://github.com/containers/skopeo.

Replace deprecated CI_BUILD_* variables in artifacts names.

• See https://gitlab.com/gitlab-org/gitlab/-/issues/352957.

Move common variables definition into a separate job/stage. Pass the variables via dotenv artifact.

• The jobs use different images with inconsistent environment, which makes running shell scripts challenging.
  • kaniko is minimalistic with just a busybox.
  • skopeo is based on Fedora.