add basic fake implementation of sdaccel-builder for testing

[CampGareth]

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Description

Review Checklist

Goals

• Does it solve the problem?

• Is it the simplest implementation of that solution?
  Does it yak shave? Does it introduce new dependencies that aren't necessary?

• Does it decrease modularity?
  Does the user of a module need to import another module to use this one?
  If we want to delete these changes, how easy is that?

• Does it clarify our domain?
  What things does it refine? What things get added? How does this pave the way for new things?
  Are things named in such a way that a domain expert can find them?

• Does it introduce non-domain concepts?
  What does the user of this need to learn outside of our domain in order to use this?

Testing

• Do we integration test changes to external services?

• Do we unit test code we can change?

[pwaller]

We have a huge number of top level directories - can it live under one of scripts, ci, build or somewhere else?

[CampGareth]

It can indeed, making the change

[pwaller]

Another couple of issues:

• It's unclear from the PR how this is actually used (who builds it? what causes testing to use it?)
• From talking to Max in person, it is meant to be tagged in the docker registry with the production build URL so that the platform picks it up when running the job under fake batch. This is inappropriate, because it comes with an associated risk of pushing the test image to production.

I suggest inlining this PR into the PR which starts using it, and fixing the latter issue by having something during the tests which builds the image (with a test image name) and something in the test which causes it to pick up this test image whilst running in development (as opposed to prod).

[CampGareth]

I don't think there's a clear need for this for now. I'll re-open it if one appears in future.

[CampGareth]

Re-opening this in the wake of #303

I think if this ends up as an image being created by docker-compose up and then gets tagged as "fake-sdaccel-builder" I think this is useful as it'll be picked up by fake-batch automatically per a commit that I've... made in another branch. Hmm. Some organisational work to be done here.