Automate upstream-downstream Dockerfile sync

At the moment we maintain 2 entirely separate Dockerfiles for each component:

• one in the GitHub upstream repo
• one in the downstream dist-git repo

Currently, changes made to the Dockerfile upstream (where each component team does their work) need to be manually applied to the downstream Dockerfile, leaving room for human error (e.g. we make a change upstream and we forget to update the downstream Dockerfile).

We need to streamline this process through automation.