fix(devops): Update AWS gateway TF (#3374)

Why:

* The previous terraform was not replacing the EC2 instance when the
`user_data` changed. This resulted in the instance only being restarted,
which meant the latest gateway image would not be pulled down. This also
highlighted the fact that the systemd unit file for the gateway was not
quite correct and if the system were to ever reboot the gateway would
not start automatically. Both issues have been corrected with this
commit.

terraform/modules/aws/gateway/main.tf

@@ -35,6 +35,7 @@ resource "aws_instance" "this" {
   associate_public_ip_address = var.associate_public_ip_address
   private_ip                  = var.private_ip
   key_name                    = var.key_name
+  user_data_replace_on_change = true
 
   user_data = templatefile("${path.module}/templates/cloud-init.yaml", {
     container_name        = local.application_name != null ? local.application_name : var.image

terraform/modules/aws/gateway/templates/cloud-init.yaml

@@ -15,15 +15,22 @@ write_files:
     content: |
       [Unit]
       Description=Start an Firezone Gateway container
+      After=docker.service
+      Requires=docker.service
 
       [Service]
       TimeoutStartSec=0
       Restart=always
+      ExecStartPre=-/usr/bin/docker stop ${container_name}
+      ExecStartPre=-/usr/bin/docker rm ${container_name}
       ExecStartPre=/usr/bin/docker pull ${container_image}
       ExecStart=/bin/sh -c 'docker run --rm --name=${container_name} --cap-add=NET_ADMIN --volume /etc/firezone --sysctl net.ipv4.ip_forward=1 --sysctl net.ipv4.conf.all.src_valid_mark=1 --sysctl net.ipv6.conf.all.disable_ipv6=0 --sysctl net.ipv6.conf.all.forwarding=1 --sysctl net.ipv6.conf.default.forwarding=1 --device="/dev/net/tun:/dev/net/tun" --env FIREZONE_NAME=$(hostname) --env FIREZONE_ID=$(echo $RANDOM$(hostname) | md5sum | head -c 20; echo;) --env-file="/etc/firezone-gateway/env" ${container_image}'
       ExecStop=/usr/bin/docker stop gateway
       ExecStopPost=/usr/bin/docker rm gateway
 
+      [Install]
+      WantedBy=multi-user.target
+
 runcmd:
   - sudo apt-get update
   - sudo apt-get install -y apt-transport-https ca-certificates curl software-properties-common
@@ -37,4 +44,4 @@ runcmd:
   - sudo systemctl stop docker
   - sudo systemctl start docker
   - sudo systemctl daemon-reload
-  - sudo systemctl start gateway.service
+  - sudo systemctl enable --now gateway.service