Docker: Warn if file access through merged dir is impossible

Even when the fallbacks via 'docker exec' work this will cause a
significant performance hit. Better inform the user.

Change-Id: If369dcc498bffa2ef422dab3e69325c4122b45ad
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/docker/dockerdevice.cpp

@@ -510,11 +510,19 @@ void DockerDevicePrivate::tryCreateLocalFileAccess()
     //LOG(proc2.commandLine().toUserOutput());
     proc.start();
     proc.waitForFinished();
-    const QByteArray out = proc.readAllStandardOutput();
-    m_mergedDir = QString::fromUtf8(out).trimmed();
+    const QString out = proc.stdOut();
+    m_mergedDir = out.trimmed();
     if (m_mergedDir.endsWith('/'))
         m_mergedDir.chop(1);
 
+    if (!QFileInfo(m_mergedDir).isWritable()) {
+        MessageManager::writeFlashing(
+            tr("Local write access to Docker container %1 unavailable through directory \"%2\".")
+                .arg(m_container, m_mergedDir)
+                    + '\n' + tr("Output: %1").arg(out)
+                    + '\n' + tr("Error: %1").arg(proc.stdErr()));
+    }
+
     m_mergedDirWatcher.addPath(m_mergedDir);
 }