Use the container sources provided by the content on the initialised
manifest instead of the blueprint to resolve containers. The container
sources on the manifest are a map keyed by the name of the pipeline that
will use the resolved containers, but the worker's container resolve job
works on a slice, so we reread the content map to get the pipeline name
(instead of taking the first payload pipeline from the image type).
This requires that there be only one pipeline that embeds containers,
which currently true of all our image types.
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