fix: handling unpadded base64 encoded docker auth field

docker-credential-desk does not pad anymore the auth field.
it is then possible to have unpadded auth field.

field might be encoded either with RawStdEncoding or StdEncoding

we now determine if it is correctly padded in order to handle
both cases.

pkg/credentialprovider/config_test.go

@@ -17,6 +17,7 @@ limitations under the License.
 package credentialprovider
 
 import (
+	"encoding/base64"
 	"encoding/json"
 	"io/ioutil"
 	"os"
@@ -208,6 +209,34 @@ func TestDecodeDockerConfigFieldAuth(t *testing.T) {
 			password: "bar",
 		},
 
+		// some test as before but with field not well padded
+		{
+			input:    "Zm9vOmJhcg",
+			username: "foo",
+			password: "bar",
+		},
+
+		// standard encoding (with padding)
+		{
+			input:    base64.StdEncoding.EncodeToString([]byte("foo:bar")),
+			username: "foo",
+			password: "bar",
+		},
+
+		// raw encoding (without padding)
+		{
+			input:    base64.RawStdEncoding.EncodeToString([]byte("foo:bar")),
+			username: "foo",
+			password: "bar",
+		},
+
+		// the input is encoded with encodeDockerConfigFieldAuth (standard encoding)
+		{
+			input:    encodeDockerConfigFieldAuth("foo", "bar"),
+			username: "foo",
+			password: "bar",
+		},
+
 		// good base64 data, but no colon separating username & password
 		{
 			input: "cGFudHM=",