Integrating CI/CD with an existing workflow

Users and operators have been moving to use containers for scientific software in order to make their workloads more portable. 
Some incompatibilities still arise based on the OS and kernel version used to build container images thus images should be built in standard environments to avoid the situation, 'the image works on my machine'. 
By adding a basic gitlab CI file to the repo, changes made to the dockerfile (on the main branch/configurable branch/branches) automatically kick off a set of stages as defined in the yml file that can build a new updated image and push it to the gitlab repo. 
This frees the user from the need to build images locally or push them anywhere. Images are also easier to find as they would show up in the container registry for the repo (https://gitlab.com/ska-telescope/src/src-workloads/container_registry) 
This work will increase the visibility of the CI features available in the gitlab ecosystem and promote the use of the same in the ART.

• A .gitlab-ci.yml file that builds and pushes an container image for a workflow task into the gitlab repo's container image registry.
• A new container image in the container registry of the repo: https://gitlab.com/ska-telescope/src/src-workloads/container_registry 
• Demo of user making changes made to the Dockerfile which result in a new image/new tag being generated in the container registry