fix a possible panic on cache

[imeoer]

newDesc.Annotations = nil
for _, k := range addAnnotations {
  newDesc.Annotations[k] = desc.Annotations[k]
}

The codes may cause buildkitd panic: assignment to entry in nil map

[imeoer]

We encountered 1-5 panics per 30,000 builds, it's related to lzay layer with image ref annotation.

[jedevc]

Nice catch 🎉

I think we want to make sure that the annotations map doesn't contain an empty map - it should either contain elements, or be nil.

[imeoer]

Nice catch 🎉

I think we want to make sure that the annotations map doesn't contain an empty map - it should either contain elements, or be nil.

Thanks for the reply, fixed.