feat: add support for configuring runner worker root device name (#1140)

## Description

These changes add support for configuring the root device name for the
Docker Machine instances. This is necessary when using an Amazon Linux
AMI where the default device name of `/dev/sda1` ends up creating a 2nd,
unused, non-root volume.


https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/device_naming.html#available-ec2-device-names

Closes #1154 

## Verification

Confirmed root device on Docker Machine instances is correctly
configured in both fleet-mode and non-fleet mode when using an AL2 AMI.
Checked this in AWS Console.

Co-authored-by: Matthias Kay <[email protected]>

Author: charlesbjohnson

Diff for: main.tf

@@ -118,6 +118,7 @@ locals {
       runners_idle_count                = var.runner_worker_docker_machine_instance.idle_count
       runners_idle_time                 = var.runner_worker_docker_machine_instance.idle_time
       runners_max_builds                = local.runners_max_builds_string
+      runners_root_device_name          = var.runner_worker_docker_machine_instance.root_device_name
       runners_root_size                 = var.runner_worker_docker_machine_instance.root_size
       runners_volume_type               = var.runner_worker_docker_machine_instance.volume_type
       runners_iam_instance_profile_name = var.runner_worker_docker_machine_role.profile_name
@@ -331,7 +332,7 @@ resource "aws_launch_template" "fleet_gitlab_runner" {
     enabled = var.runner_worker_docker_machine_instance.monitoring
   }
   block_device_mappings {
-    device_name = "/dev/sda1"
+    device_name = var.runner_worker_docker_machine_instance.root_device_name
 
     ebs {
       volume_size = var.runner_worker_docker_machine_instance.root_size

Diff for: template/runner-config.tftpl

@@ -64,6 +64,7 @@ listen_address = "${prometheus_listen_address}"
       "amazonec2-use-ebs-optimized-instance=${runners_ebs_optimized}",
       "amazonec2-monitoring=${runners_monitoring}",
       "amazonec2-iam-instance-profile=%{ if runners_iam_instance_profile_name != "" }${runners_iam_instance_profile_name}%{ else }${runners_instance_profile}%{ endif ~}",
+      "amazonec2-device-name=${runners_root_device_name}",
       "amazonec2-root-size=${runners_root_size}",
       "amazonec2-volume-type=${runners_volume_type}",
       "amazonec2-userdata=%{~ if runners_userdata != "" ~}/etc/gitlab-runner/runners_userdata.sh%{~ endif ~}",

Diff for: variables.tf

@@ -675,6 +675,7 @@ variable "runner_worker_docker_machine_instance" {
     monitoring = Enable detailed monitoring for the Runner Worker.
     name_prefix = Set the name prefix and override the `Name` tag for the Runner Worker.
     private_address_only = Restrict Runner Worker to the use of a private IP address. If `runner_instance.use_private_address_only` is set to `true` (default), `runner_worker_docker_machine_instance.private_address_only` will also apply for the Runner.
+    root_device_name = The name of the root volume for the Runner Worker.
     root_size = The size of the root volume for the Runner Worker.
     start_script = Cloud-init user data that will be passed to the Runner Worker. Should not be base64 encrypted.
     subnet_ids = The list of subnet IDs to use for the Runner Worker when the fleet mode is enabled.
@@ -693,6 +694,7 @@ variable "runner_worker_docker_machine_instance" {
     monitoring                 = optional(bool, false)
     name_prefix                = optional(string, "")
     private_address_only       = optional(bool, true)
+    root_device_name           = optional(string, "/dev/sda1")
     root_size                  = optional(number, 8)
     start_script               = optional(string, "")
     subnet_ids                 = optional(list(string), [])