Use cachekey not digest for COPY --from src

* use the cachekey of the src stage rather than the digest
for COPY --from commands as they are reproducible unlike digests
* track digest to cache keys and stage indexes to digest
* add extra debug logging for troubleshooting cachekey building issues
* convert Sha256 hashes to hex encoded strings rather than plain strings
for easier human reading

pkg/executor/composite_cache.go

@@ -18,6 +18,7 @@ package executor
 
 import (
 	"crypto/sha256"
+	"fmt"
 	"os"
 	"path/filepath"
 	"strings"
@@ -75,7 +76,7 @@ func (s *CompositeCache) AddPath(p string) error {
 		return err
 	}
 
-	s.keys = append(s.keys, string(sha.Sum(nil)))
+	s.keys = append(s.keys, fmt.Sprintf("%x", sha.Sum(nil)))
 	return nil
 }
 
@@ -98,5 +99,5 @@ func HashDir(p string) (string, error) {
 		return "", err
 	}
 
-	return string(sha.Sum(nil)), nil
+	return fmt.Sprintf("%x", sha.Sum(nil)), nil
 }