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

CHANGELOG.md

@@ -1,7 +1,8 @@
 ## 1.1.3 (Unreleased)
 
 BUG FIXES:
-csi: fixed a CLI panic when formatting `volume status` with `-verbose` flag [[GH-10818](https://github.com/hashicorp/nomad/issues/10818)]
+* csi: Fixed a CLI panic when formatting `volume status` with `-verbose` flag [[GH-10818](https://github.com/hashicorp/nomad/issues/10818)]
+* 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. [[GH-10823](https://github.com/hashicorp/nomad/issues/10823)]
 
 ## 1.1.2 (June 22, 2021)
 

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