Allow container builds to use pre-built pkgs defined by target

[MorrisLaw]

What this PR does / why we need it:
This PR adds functionality that allows a user to pass in a pre-built package via a target and then uses that same package when building the container. Allows for users to separate package builds from container builds.

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):
Fixes #597

[DannyBrito]

how would this look like when calling for the build?
something like docker build -f <spec> --target="<path-to-rpm>" .?

[MorrisLaw]

how would this look like when calling for the build? something like docker build -f <spec> --target="<path-to-rpm>" .?

Good question. I think something like this would be the workflow:

1. Build the pkg first:
   docker buildx build --file=spec.yaml --target=rpm --output=type=local,dest=./pkgs .

2. Build the container using the prebuilt pkg:
   docker buildx build --file=spec.yaml --target=container --build-context=rpm-pkg=./pkgs .

If you look in the test, it's also how I'm including it: withBuildContext. Here are the dalec docs as well: https://azure.github.io/dalec/targets/#named-build-contexts

cc/ @cpuguy83 if I'm incorrect or missing anything

[MorrisLaw]

Should be good now after addressing the timeout issue: #671