Initialise the manifest only once in the enqueue functions
(ImageType.Manifest()) and pass it to the manifest generation function
with the workers and the dependency IDs. The function is renamed from
generateManifest() to serializeManifest() to reflect its new
functionality more accurately. The arguments to the function have also
been trimmed since we no longer need the image type, blueprint, and
image options.
The new functionality of the function is to collect all the resolved
content from the workers and serialize the manifest object.
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