Add GitHub Action to build validator and driver images

[philsmart]

• Add buildx image building function
• see https://github.com/ukf/ukf-testbed/issues/5

I added each docker build command to the Action, rather than calling the build-image scripts. It seems a tighter integration into the Action workflow. But I can change that if undesirable.

[philsmart]

Updated the Action to continue a single job with multiple steps. Which, for this task, is actually faster than the parallel jobs approach (it only needs to setup one container) as @iay suggested.

[iay]

"Premature optimization is the root of all evil"