[WIP] add prebuild work images

[DannyBrito]

What this PR does / why we need it:
Initial work to add pre-build images for mariner, azlinux worker images.

Fixes # #177

Questions:

1. Would these work images need to be versioned? If so, would these follow same versioning of dalec tags?
2. Would we need to add a ci for these images?

[pmengelbert]

Can you update the PR description with some detail on what problem this is solving?

[pmengelbert]

See above comment

[cpuguy83]

In #328 we added support for:

1. Supplying a custom worker image by providing a specifically named build context (name depends on the target)
2. Building/exporting just the worker image as a build target, e.g. mariner2/worker

With these we have these pre-cached worker images without having to maintain a custom set in the repo (though we still could do this, just using the new build endpoints).