We can use cross repository push feature to reuse the existing blobs in
the same registry. Before make push fast, we know where the blob comes
from.
Use the `containerd.io/distribution.source. = [,]` as label format. For
example, the blob is downloaded by the docker.io/library/busybox:latest
and the label will be
containerd.io/distribution.source.docker.io = library/busybox
If the blob is shared by different repos in the same registry, the repo
name will be appended, like:
containerd.io/distribution.source.docker.io = library/busybox,x/y
NOTE:
1. no need to apply for legacy docker image schema1.
2. the concurrent fetch actions might miss some repo names in label, but
it is ok.
3. it is optional. no need to add label if the engine only uses images
not push.
Signed-off-by: Wei Fu <fuweid89@gmail.com>
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