containerd: Implement push

[vvoland]

Make docker push push

Signed-off-by: Paweł Gronowski pawel.gronowski@docker.com

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[rumpl]

@vvoland There is a conflict :)

[vvoland]

Thanks for the notice! Rebased :)

[thaJeztah]

When upstreaming:

• depends on / needs to go after containerd integration: produce progress events polling ctrd's content.Store moby/moby#43819
• but, we may want to squash all fixes related to "common" progress (push/pull) code into a single commit

combine with one or more of;

• containerd: Implement push #15
• Push progress #16
• c8d/progress: Fix progress not ending #73
• c8d/push: Push lazy blobs with distribution source labels #74
• progress fix-up from c8d/push: Fetch missing resources that can't be mounted #80