fix: various lint issues

Originally found by @jkroepke in: https://github.com/project-zot/zot/actions/runs/10056774230/job/27796324933?pr=2556
And #2556 in general

This commit covers:
- (canonicalheader) capitalization of Docker-Content-Digest header
- (protogetter) the proto getters were not used, they check for nil pointers, we should switch to using them
- (zerologlint) fix the false positive

Signed-off-by: Andrei Aaron <aaaron@luxoft.com>

pkg/api/session.go

@@ -89,7 +89,7 @@ func SessionLogger(ctlr *Controller) mux.MiddlewareFunc {
 			clientIP := request.RemoteAddr
 			method := request.Method
 			headers := map[string][]string{}
-			log := logger.Info()
+			log := logger.Info() //nolint: zerologlint // false positive, the Msg call is below
 			for key, value := range request.Header {
 				if key == "Authorization" { // anonymize from logs
 					s := strings.SplitN(value[0], " ", 2) //nolint:gomnd

pkg/cli/client/client.go

@@ -307,7 +307,7 @@ func fetchImageIndexStruct(ctx context.Context, job *httpJob) (*imageStruct, err
 		return nil, err
 	}
 
-	indexDigest := header.Get("docker-content-digest")
+	indexDigest := header.Get("Docker-Content-Digest")
 
 	indexSize, err := strconv.ParseInt(header.Get("Content-Length"), 10, 64)
 	if err != nil {
@@ -398,7 +398,7 @@ func fetchManifestStruct(ctx context.Context, repo, manifestReference string, se
 		return common.ManifestSummary{}, err
 	}
 
-	manifestDigest := header.Get("docker-content-digest")
+	manifestDigest := header.Get("Docker-Content-Digest")
 	configDigest := manifestResp.Config.Digest.String()
 
 	configContent, err := fetchConfig(ctx, repo, configDigest, searchConf, username, password)

pkg/cli/client/utils_internal_test.go

@@ -265,7 +265,7 @@ func TestDoHTTPRequest(t *testing.T) {
 						vars := mux.Vars(req)
 
 						if vars["reference"] == "indexRef" {
-							writer.Header().Add("docker-content-digest", godigest.FromString("t").String())
+							writer.Header().Add("Docker-Content-Digest", godigest.FromString("t").String())
 							_, err := writer.Write([]byte(`
 								{
 									"manifests": [
@@ -594,7 +594,7 @@ func TestDoJobErrors(t *testing.T) {
 					Route: "/v2/{name}/manifests/{reference}",
 					HandlerFunc: func(w http.ResponseWriter, r *http.Request) {
 						w.Header().Add("Content-Type", ispec.MediaTypeImageIndex)
-						w.Header().Add("docker-content-digest", godigest.FromString("t").String())
+						w.Header().Add("Docker-Content-Digest", godigest.FromString("t").String())
 
 						_, err := w.Write([]byte(""))
 						if err != nil {
@@ -609,7 +609,7 @@ func TestDoJobErrors(t *testing.T) {
 						vars := mux.Vars(req)
 
 						if vars["reference"] == "indexRef" {
-							writer.Header().Add("docker-content-digest", godigest.FromString("t").String())
+							writer.Header().Add("Docker-Content-Digest", godigest.FromString("t").String())
 
 							_, err := writer.Write([]byte(`{"manifests": [{"digest": "manifestRef"}]}`))
 							if err != nil {
@@ -618,7 +618,7 @@ func TestDoJobErrors(t *testing.T) {
 						}
 
 						if vars["reference"] == "manifestRef" {
-							writer.Header().Add("docker-content-digest", godigest.FromString("t").String())
+							writer.Header().Add("Docker-Content-Digest", godigest.FromString("t").String())
 
 							_, err := writer.Write([]byte(`{"config": {"digest": "confDigest"}}`))
 							if err != nil {