feat(server): Order layers in image manifest based on merge rating

Image layers in manifests are now sorted in a stable (descending)
order based on their merge rating, meaning that layers more likely to
be shared between images come first.

The reason for this change is Docker's handling of image layers on
overlayfs2: Images are condensed into a single representation on disk
after downloading.

Due to this Docker will constantly redownload all layers that are
applied in a different order in different images (layer order matters
in imperatively created images), based on something it calls the
'ChainID'.

Sorting the layers this way raises the likelihood of a long chain of
matching layers at the beginning of an image.

This relates to #39.

tools/nixery/server/builder/builder.go

@@ -250,6 +250,7 @@ func prepareLayers(ctx context.Context, s *State, image *Image, result *ImageRes
 			if err != nil {
 				return nil, err
 			}
+			entry.MergeRating = l.MergeRating
 
 			go cacheLayer(ctx, s, l.Hash(), *entry)
 			entries = append(entries, *entry)