Limit size of additional label for avoiding preparation failure
Limit size of additional label for avoiding preparation failure

In containerd, there is a size limit for label size (4096 chars).
If an image has many layers (> (4096-43)/72 > 55),
`containerd.io/snapshot/remote/stargz.layers` will hit the limit of
label size and the remote snapshot preparation will fail.
This commit fixes this by limiting the size of the label.

Signed-off-by: ktock <72a466a7d91579e23b8af87d6766ee221241ce87@gmail.com>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc