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instead of appending image tags, prepend them - this generally produces better results

the buffered value is going into the hasher eventually so write directly
to the hasher instead

default client already handles proxy

- consistent format by using text handler for debug and non-debug
- truncate source file to just the file name

if create overrides a manifest, first add the older manifest's layers to
the delete map so they can be cleaned up

prefer using req.Model and fallback to req.Name

A few obvious levels were adjusted, but generally everything mapped to "info" level.

if a download part is inactive for some time, restart it

this fixes a subtle bug with makeRequestWithRetry where an HTTP status
error on a retried request will potentially not return the right err