Docker: Switch Linux file handling to docker --exec by default

Also on Linux, where there's actually a choice.

It's slower, but less fragile when it comes to user ids
and/or file system permissions.

Change-Id: I811612fcff65fad50be18bed21150f6ca7fae93c
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/docker/dockerdevice.cpp

@@ -396,6 +396,13 @@ public:
             data.useLocalUidGid = on;
         });
 
+        // This tries to find the directory in the host file system that corresponds to the
+        // docker container root file system, which is a merge of the layers from the
+        // container image and the volumes mapped using -v on container startup.
+        //
+        // Accessing files there is much faster than using 'docker exec', but conceptually
+        // only works on Linux, and is restricted there by proper matching of user
+        // permissions between host and container.
         m_usePathMapping = new QCheckBox(tr("Use local file path mapping"));
         m_usePathMapping->setToolTip(tr("Maps docker filesystem to a local directory."));
         m_usePathMapping->setChecked(data.useFilePathMapping);
@@ -960,8 +967,10 @@ void DockerDevicePrivate::tryCreateLocalFileAccess()
         }
         return;
     }
+
     if (!DockerPlugin::isDaemonRunning().value_or(false))
         return;
+
     QtcProcess proc;
     proc.setCommand({"docker", {"inspect", "--format={{.GraphDriver.Data.MergedDir}}", m_container}});
     LOG(proc.commandLine().toUserOutput());

src/plugins/docker/dockerdevice.h

@@ -44,7 +44,7 @@ public:
     QString tag;
     QString size;
     bool useLocalUidGid = true;
-    bool useFilePathMapping = true;
+    bool useFilePathMapping = false;
     QStringList mounts;
 };