refactor(digests): standardise representation of digests to digest.Digest (#898)

- Digests were represented by different ways
  - We needed a uniform way to represent the digests and enforce a format
  - also replace usage of github.com/google/go-containerregistry/pkg/v1
    with github.com/opencontainers/image-spec/specs-go/v1

Signed-off-by: Laurentiu Niculae <niculae.laurentiu1@gmail.com>
(cherry picked from commit 96b2f29d6d57070a913ce419149cd481c0723815)
(cherry picked from commit 3d41b583daea654c98378ce3dcb78937d71538e8)

Co-authored-by: Laurentiu Niculae <niculae.laurentiu1@gmail.com>

pkg/cli/client.go

@@ -17,7 +17,6 @@ import (
 	"os"
 	"path/filepath"
 	"strconv"
-	"strings"
 	"sync"
 	"time"
 
@@ -272,11 +271,8 @@ func (p *requestsPool) doJob(ctx context.Context, job *manifestJob) {
 		p.outputCh <- stringResult{"", err}
 	}
 
-	digest := header.Get("docker-content-digest")
-	digest = strings.TrimPrefix(digest, "sha256:")
-
+	digestStr := header.Get("docker-content-digest")
 	configDigest := job.manifestResp.Config.Digest
-	configDigest = strings.TrimPrefix(configDigest, "sha256:")
 
 	var size uint64
 
@@ -289,7 +285,7 @@ func (p *requestsPool) doJob(ctx context.Context, job *manifestJob) {
 			layers,
 			layer{
 				Size:   entry.Size,
-				Digest: strings.TrimPrefix(entry.Digest, "sha256:"),
+				Digest: entry.Digest,
 			},
 		)
 	}
@@ -298,7 +294,7 @@ func (p *requestsPool) doJob(ctx context.Context, job *manifestJob) {
 	image.verbose = *job.config.verbose
 	image.RepoName = job.imageName
 	image.Tag = job.tagName
-	image.Digest = digest
+	image.Digest = digestStr
 	image.Size = strconv.Itoa(int(size))
 	image.ConfigDigest = configDigest
 	image.Layers = layers