Extend Zuul pipeline to allow jobs reusing a Docker images built in a preceding job of the same pipeline

[ipatini]

EMS component comprises a core service (called ems-core), and a project-specific plugin (called ems-nebulous). The former module will compile and build a Docker image with the ems-core, while the latter will extend this Docker image (by adding a new file and set a few ENV variables), and produce the final EMS Docker image for Nebulous.
To this end i would like to ask extending Zuul pipeline to support this case (i.e. a job being able to use a previously built Docker image).

[Radosław Piliszek (yoctozepto)]

This is already happening in the deployment jobs. Nonetheless, the image building jobs currently assume they always build from scratch or some other already existing image.