Add capability to automatically push README.md files to dockerhub as part of deployment

Issue/feature addressed

This MR automates deployment of the README.md files included in docker/ directories when they are pushed up to Dockerhub.

Proposed solution

To enable this we use the docker-pushrm image, which just extracts the supplied README.md and pushes this up to Dockerhub using their API.

Implementation

These are added as new jobs on Gitlab CI. As of gitlab 14.x we can use needs: to restrict jobs even within the same stage, so this MR simplifies staging a bit to remove the package-build-dev stage.

Checklist

• Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• No extraneous files have been added (e.g. compiler output or test data files).