use docker exec instead of dockerode exec in mysql

this way we can check the exit code of the exec process.
preivously, we were only wait for the stream to end.

src/docker.js

@@ -403,10 +403,13 @@ function execContainer(containerId, cmd, input, callback) {
     callback = once(callback); // ChildProcess exit may or may not be called after error
 
     var cp = spawn('/usr/bin/docker', [ 'exec', '-i', containerId  ].concat(cmd));
-    cp.on('error', callback);
+    var chunks = [ ];
+    cp.stdout.on('data', function (chunk) { chunks.push(chunk); });
+
+    cp.on('error', function (error) { console.log(error); callback(error); });
     cp.on('exit', function (code, signal) {
         debug('execContainer code: %s signal: %s', code, signal);
-        if (!callback.called) callback(code ? 'Failed with status ' + code : null, process.stdout);
+        if (!callback.called) callback(code ? 'Failed with status ' + code : null, Buffer.concat(chunks));
     });
 
     cp.stderr.pipe(process.stderr);