images: Export published image refs as dotenv report

GitLab CI allows one to export environment variables from one job to
another through the use of the `dotenv` report.

See https://docs.gitlab.com/ee/ci/variables/index.html#pass-an-environment-variable-to-another-job

Use a `dotenv` report to export variables that contain information about
the published image. Each variable is prefixed by the name of the job
which published the image with `-` replaced by `_`.

This should allow, among other use cases, a job to directly run an image
built in a previous stage using native GitLab CI constructs.

    build-foo:
      stage: build
      extends: [.kokkuri:build-and-publish-image]
      variables:
        BUILD_VARIANT: foo

    run-foo:
      stage: run
      image: ${BUILD_FOO_IMAGE_REF}
      script:
        - echo "doing stuff with the image from build-foo"