Synchronize docker config loading

This prevents data races during testing. The root cause
are unsecured globals in the docker cli lib.

Signed-off-by: Jan Boehm <6552713+jeloba@users.noreply.github.com>

pkg/image/containerdregistry/resolver.go

@@ -5,6 +5,7 @@ import (
 	"crypto/x509"
 	"net"
 	"net/http"
+	"sync"
 	"time"
 
 	"github.com/containerd/containerd/remotes"
@@ -85,7 +86,13 @@ func credential(cfg *configfile.ConfigFile) func(string) (string, string, error)
 	}
 }
 
+// protects against a data race inside the docker CLI
+var configMutex sync.Mutex
+
 func loadConfig(dir string) (*configfile.ConfigFile, error) {
+	configMutex.Lock()
+	defer configMutex.Unlock()
+
 	if dir == "" {
 		dir = config.Dir()
 	}