logs: use %o to format error

otherwise, they are printed as multi-line and this messes up tail+date formatting

src/dockerproxy.js

@@ -54,7 +54,7 @@ function attachDockerRequest(req, res, next) {
         // Force node to send out the headers, this is required for the /container/wait api to make the docker cli proceed
         res.write(' ');
 
-        dockerResponse.on('error', function (error) { debug('dockerResponse error:', error); });
+        dockerResponse.on('error', function (error) { debug('dockerResponse error: %o', error); });
         dockerResponse.pipe(res, { end: true });
     });