add /auth for docker compatibility

This endpoint just validates credentials:
https://github.com/moby/moby/blob/v20.10.4/api/swagger.yaml#L7936-L7977

Fixes: #9564
Signed-off-by: troyready <troy@troyready.com>

pkg/api/server/swagger.go

@@ -226,3 +226,12 @@ type swagSystemPruneReport struct {
 		entities.SystemPruneReport
 	}
 }
+
+// Auth response
+// swagger:response SystemAuthResponse
+type swagSystemAuthResponse struct {
+	// in:body
+	Body struct {
+		entities.AuthReport
+	}
+}