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

.changelog/10823.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+docker: Moved the generated `/etc/hosts` file's mount source to the allocation directory so that it can be shared between tasks of an allocation.
+```

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

website/content/docs/upgrade/upgrade-specific.mdx

@@ -13,6 +13,16 @@ upgrade. However, specific versions of Nomad may have more details provided for
 their upgrades as a result of new features or changed behavior. This page is
 used to document those details separately from the standard upgrade flow.
 
+## Nomad 1.1.3
+
+#### Docker Driver
+
+Starting in Nomad 1.1.2, task groups with `network.mode = "bridge"` generated
+a hosts file in Docker containers. This generated hosts file was bind-mounted
+from the task directory to `/etc/hosts` within the task. In Nomad 1.1.3 the
+source for the bind mount was moved to the allocation directory so that it is
+shared between all tasks in an allocation.
+
 ## Nomad 1.1.0
 
 #### Enterprise licenses