docker: move host path for hosts file mount to alloc dir

In Nomad 1.1.1 we generate a hosts file based on the Nomad-owned network
namespace, rather than using the default hosts file from the pause
container. This hosts file should be shared between tasks in the same
allocation so that tasks can update the file and have the results propagated
between tasks.

drivers/docker/driver.go

@@ -962,7 +962,7 @@ func (d *Driver) createContainerConfig(task *drivers.TaskConfig, driverConfig *T
 	// that comes from the pause container
 	if task.NetworkIsolation != nil && driverConfig.NetworkMode == "" {
 		etcHostMount, err := hostnames.GenerateEtcHostsMount(
-			task.TaskDir().Dir, task.NetworkIsolation, driverConfig.ExtraHosts)
+			task.AllocDir, task.NetworkIsolation, driverConfig.ExtraHosts)
 		if err != nil {
 			return c, fmt.Errorf("failed to build mount for /etc/hosts: %v", err)
 		}

drivers/shared/hostnames/mount.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"net"
+	"os"
 	"path/filepath"
 	"strings"
 
@@ -56,9 +57,14 @@ ff02::3 ip6-allhosts
 	}
 
 	path := filepath.Join(taskDir, "hosts")
-	err := ioutil.WriteFile(path, []byte(content.String()), 0644)
-	if err != nil {
-		return nil, err
+
+	// tasks within an alloc should be able to share and modify the file, so
+	// only write to it if it doesn't exist
+	if _, err := os.Stat(path); os.IsNotExist(err) {
+		err := ioutil.WriteFile(path, []byte(content.String()), 0644)
+		if err != nil {
+			return nil, err
+		}
 	}
 
 	// Note that we're not setting readonly. The file is in the task dir