Pass labels to containerd

[WeiZhang555]

CreateContainerRequest contains labels field but docker never passes
labels to containerd, which makes it impossible to set labels for
containerd.

This commit will add label support for containerd.

Signed-off-by: Zhang Wei zhangwei555@huawei.com

[crosbymichael]

Right now we have labels in containerd for internal use and we don't want to propagate user labels to it at this time.

The use case that we want to work out is if we are using containers as plugins or to bootstrap different functionality we want to be able to use labels at the containerd level to specify what is a user container, what is a system container, and what is a plugin container.

This is still being worked out so it is best that we do not propagate the user defined labels at this time.

Thanks!

[WeiZhang555]

@crosbymichael

So this is kept for specifying types of container, user container/system container, seems concept from RancherOS.

TBH, my scenario is that I'm trying to make a "custom" containerd, and make it possible to call runv from docker with this custom containerd, but there's no way to pass the kernel configs(see opencontainers/runtime-spec#405), so I want to reuse the labels to pass kernel related configs to my custom containerd.

I can accept that if you say this is kept for special purpose, but please let me know if you change your mind, now I'll try to find a different way to do that 😄