Fix inconsistent task env setting

Consolidate task environment building in GetTaskEnv since it can
determine what kind of filesystem isolation is used.

This means drivers no longer have to manipulate task environment paths.

client/driver/docker.go

@@ -352,10 +352,6 @@ func (d *DockerDriver) FSIsolation() cstructs.FSIsolation {
 }
 
 func (d *DockerDriver) Prestart(ctx *ExecContext, task *structs.Task) error {
-	// Set environment variables.
-	d.taskEnv.SetAllocDir(allocdir.SharedAllocContainerPath).
-		SetTaskLocalDir(allocdir.TaskLocalContainerPath).SetSecretsDir(allocdir.TaskSecretsContainerPath).Build()
-
 	driverConfig, err := NewDockerDriverConfig(task, d.taskEnv)
 	if err != nil {
 		return err

client/driver/driver.go

@@ -164,10 +164,22 @@ func GetTaskEnv(taskDir *allocdir.TaskDir, node *structs.Node,
 		SetEnvvars(task.Env).
 		SetTaskName(task.Name)
 
-	if taskDir != nil {
+	// Vary paths by filesystem isolation used
+	drv, err := NewDriver(task.Driver, NewEmptyDriverContext())
+	if err != nil {
+		return nil, err
+	}
+	switch drv.FSIsolation() {
+	case cstructs.FSIsolationNone:
+		// Use host paths
 		env.SetAllocDir(taskDir.SharedAllocDir)
 		env.SetTaskLocalDir(taskDir.LocalDir)
 		env.SetSecretsDir(taskDir.SecretsDir)
+	default:
+		// filesystem isolation; use container paths
+		env.SetAllocDir(allocdir.SharedAllocContainerPath)
+		env.SetTaskLocalDir(allocdir.TaskLocalContainerPath)
+		env.SetSecretsDir(allocdir.TaskSecretsContainerPath)
 	}
 
 	if task.Resources != nil {

client/driver/driver_test.go

@@ -130,7 +130,7 @@ func testDriverContexts(t *testing.T, task *structs.Task) *testContext {
 func TestDriver_GetTaskEnv(t *testing.T) {
 	task := &structs.Task{
 		Name:   "Foo",
-		Driver: "mock",
+		Driver: "mock_driver",
 		Env: map[string]string{
 			"HELLO": "world",
 			"lorem": "ipsum",
@@ -155,7 +155,9 @@ func TestDriver_GetTaskEnv(t *testing.T) {
 	alloc := mock.Alloc()
 	alloc.Job.TaskGroups[0].Tasks[0] = task
 	alloc.Name = "Bar"
-	env, err := GetTaskEnv(nil, nil, task, alloc, testConfig(), "")
+	conf := testConfig()
+	allocDir := allocdir.NewAllocDir(testLogger(), filepath.Join(conf.AllocDir, alloc.ID))
+	env, err := GetTaskEnv(allocDir.NewTaskDir(task.Name), nil, task, alloc, conf, "")
 	if err != nil {
 		t.Fatalf("GetTaskEnv() failed: %v", err)
 	}