Add a new `containers` section that can be used to request the
embedding of containers into images. The only requirement is
the source property to specify where to fetch the container from.
This suppports specifying the digest of the container or the tag.
In case none is given it defaults to the `latest` tag. The `Name`
field can be used to optionally specify a name to use inside the
image.
NB: currently no tools or apis support container resolution yet.
This follows in the next commits.
This is the first step to support embedding container images. Here
we add the `containers []container.Spec` argument to supply images
with resolved container specifications. For now all distros will
return an error in case a container is actually supplied since none
of them currently support embedding containers. NB: also no apis or
tools will actually resolve containers.
The package sets for an image can depend on the blueprint, and
by the same logic there is no reason it should not be able to
depend on the image options.
This is so far a non-functional change, but makes a follow-up
commit simpler (though still without actually depending on
the image options to compute the package sets).
The value doesn't represent the worker name, just the top-level cache
directory for a job. It's useful for separating caches and making the
generation faster, but it's not necessary to return from the function.
