Add support for --init to docker driver.

.changelog/11331.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+driver/docker: Added support for Docker's `--init` parameter
+```

drivers/docker/config.go

@@ -354,6 +354,7 @@ var (
 		"extra_hosts":        hclspec.NewAttr("extra_hosts", "list(string)", false),
 		"force_pull":         hclspec.NewAttr("force_pull", "bool", false),
 		"hostname":           hclspec.NewAttr("hostname", "string", false),
+		"init":               hclspec.NewAttr("init", "bool", false),
 		"interactive":        hclspec.NewAttr("interactive", "bool", false),
 		"ipc_mode":           hclspec.NewAttr("ipc_mode", "string", false),
 		"ipv4_address":       hclspec.NewAttr("ipv4_address", "string", false),
@@ -434,6 +435,7 @@ type TaskConfig struct {
 	ExtraHosts        []string           `codec:"extra_hosts"`
 	ForcePull         bool               `codec:"force_pull"`
 	Hostname          string             `codec:"hostname"`
+	Init              bool               `codec:"init"`
 	Interactive       bool               `codec:"interactive"`
 	IPCMode           string             `codec:"ipc_mode"`
 	IPv4Address       string             `codec:"ipv4_address"`

drivers/docker/driver.go

@@ -853,6 +853,11 @@ func (d *Driver) createContainerConfig(task *drivers.TaskConfig, driverConfig *T
 		hostConfig.CPUSetCPUs = driverConfig.CPUSetCPUs
 	}
 
+	// Enable tini (docker-init) init system.
+	if driverConfig.Init {
+		hostConfig.Init = driverConfig.Init
+	}
+
 	// Calculate CPU Quota
 	// cfs_quota_us is the time per core, so we must
 	// multiply the time by the number of cores available

drivers/docker/driver_test.go

@@ -1526,6 +1526,31 @@ func TestDockerDriver_DNS(t *testing.T) {
 
 }
 
+func TestDockerDriver_Init(t *testing.T) {
+	if !tu.IsCI() {
+		t.Parallel()
+	}
+	testutil.DockerCompatible(t)
+	if runtime.GOOS == "windows" {
+		t.Skip("Windows does not support init.")
+	}
+
+	task, cfg, ports := dockerTask(t)
+	defer freeport.Return(ports)
+
+	cfg.Init = true
+	require.NoError(t, task.EncodeConcreteDriverConfig(cfg))
+
+	client, d, handle, cleanup := dockerSetup(t, task, nil)
+	defer cleanup()
+	require.NoError(t, d.WaitUntilStarted(task.ID, 5*time.Second))
+
+	container, err := client.InspectContainer(handle.containerID)
+	require.NoError(t, err)
+
+	require.Equal(t, cfg.Init, container.HostConfig.Init)
+}
+
 func TestDockerDriver_CPUSetCPUs(t *testing.T) {
 	if !tu.IsCI() {
 		t.Parallel()

website/content/docs/drivers/docker.mdx

@@ -124,6 +124,16 @@ config {
   launching more than one of a task (using `count`) with this option set, every
   container the task starts will have the same hostname.
 
+- `init` - (Optional) `true` or `false` (default). Enable init (tini) system when
+   launching your container. When enabled, an init process will be used as the PID1
+   in the container. Specifying an init process ensures the usual responsibilities
+   of an init system, such as reaping zombie processes, are performed inside the
+   created container.
+
+   The default init process used is the first `docker-init` executable found in the
+   system path of the Docker daemon process. This `docker-init` binary, included in
+   the default installation, is backed by [tini][tini].
+
 - `interactive` - (Optional) `true` or `false` (default). Keep STDIN open on
   the container.
 
@@ -1152,6 +1162,7 @@ Windows is relatively new and rapidly evolving you may want to consult the
 [cap_drop]: /docs/drivers/docker#cap_drop
 [no_net_raw]: /docs/upgrade/upgrade-specific#nomad-1-1-0-rc1-1-0-5-0-12-12
 [upgrade_guide_extra_hosts]: /docs/upgrade/upgrade-specific#docker-driver
+[tini]: https://github.com/krallin/tini
 [docker_caps]: https://docs.docker.com/engine/reference/run/#runtime-privilege-and-linux-capabilities
 [allow_caps]: /docs/drivers/docker#allow_caps
 [Connect]: /docs/job-specification/connect