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.


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.

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

      stage: run
      image: ${BUILD_FOO_IMAGE_REF}
        - echo "doing stuff with the image from build-foo"
2 jobs for review/export-published-image in 7 seconds (queued for 12 seconds)