Backport containerd patches from upstream (fix FTBFS moby/buildkit)

As is almost expected, upstream can't help but build against the master
branch, instead of sticking to a release branch.

In this case, moby/buildkit vendors containerd
v1.4.1-0.20200903181227-d4e78200d6da, meaning that it won't build
against containerd v1.4.x.

~~~~ FTBFS
vendor/github.com/moby/buildkit/util/resolver/authorizer.go:16:2: cannot find package "github.com/containerd/containerd/remotes/docker/auth" in any of:
	/usr/src/docker/.gopath/src/github.com/docker/docker/vendor/github.com/containerd/containerd/remotes/docker/auth (vendor tree)
	/usr/lib/go-1.15/src/github.com/containerd/containerd/remotes/docker/auth (from $GOROOT)
	/usr/src/docker/.gopath/src/github.com/containerd/containerd/remotes/docker/auth (from $GOPATH)
vendor/github.com/moby/buildkit/util/resolver/authorizer.go:17:2: cannot find package "github.com/containerd/containerd/remotes/errors" in any of:
	/usr/src/docker/.gopath/src/github.com/docker/docker/vendor/github.com/containerd/containerd/remotes/errors (vendor tree)
	/usr/lib/go-1.15/src/github.com/containerd/containerd/remotes/errors (from $GOROOT)
	/usr/src/docker/.gopath/src/github.com/containerd/containerd/remotes/errors (from $GOPATH)
~~~~