Add Key() to LayeredMap and Snapshotter

This will return a string representaiton of the current filesystem to be
used with caching.

Whenever a file is explictly added (via ADD or COPY), it will be stored
in "added" in the LayeredMap. The file will map to a hash created by
CacheHasher (which doesn't take into account mtime, since that will be
different with every build, making the cache useless)

Key() will returns a sha of the added files which will be used in
determining the overall cache key for a command.

pkg/executor/build.go

@@ -62,7 +62,7 @@ func DoBuild(opts *options.KanikoOptions) (v1.Image, error) {
 		if err := util.GetFSFromImage(constants.RootDir, sourceImage); err != nil {
 			return nil, err
 		}
-		l := snapshot.NewLayeredMap(hasher)
+		l := snapshot.NewLayeredMap(hasher, util.CacheHasher())
 		snapshotter := snapshot.NewSnapshotter(l, constants.RootDir)
 		// Take initial snapshot
 		if err := snapshotter.Init(); err != nil {