With this PR, we introduce the concept of image handlers. They support
walking a tree of image resource descriptors for doing various tasks
related to processing them. Handlers can be dispatched sequentially or
in parallel and can be stacked for various effects.
The main functionality we introduce here is parameterized fetch without
coupling format resolution to the process itself. Two important
handlers, `remotes.FetchHandler` and `image.ChildrenHandler` can be
composed to implement recursive fetch with full status reporting. The
approach can also be modified to filter based on platform or other
constraints, unlocking a lot of possibilities.
This also includes some light refactoring in the fetch command, in
preparation for submission of end to end pull.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
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