[ci] Docker image building as part of CI

Context

Currently, Docker images are built manually, any updates to be performed in the images needs to be manually done by the maintainer.

Expected behavior

Docker image generation and pushing to docker hub becomes part of the CI pipeline. It can be added as the first stage which only builds and pushes if there is a change in the Dockerfile else this stage can be skipped.

Current Behavior

Currently, this docker image generation is done manually. The Dockerfile only exists for Ubuntu 16.04, for the other images isn't available on the repo.

Describe the solution you'd like

The first stage of CI is Docker image generation from the Dockerfiles in the repo.
The CI check's to see if Dockerfile was updated (using git diff) if it was it builds and pushes the new image to docker hub else the stage is skipped.

Additional context
Have already implemented some of the above-mentioned stuff here here. (CI Results)

Start with .ci/azure-pipelines/build-ubuntu.yaml and the repository's existing Dockerfile; compare the linked CI results with the current pipeline behavior. Trace how git diff would detect Dockerfile changes and how Docker Hub publishing is configured. Done means image generation and pushing are part of CI when relevant Dockerfiles change, while the stage is skipped otherwise.