Add support for --init to docker driver.

[shishir-a412ed]

Fixes #2719

Signed-off-by: Shishir Mahajan smahajan@roblox.com

[shishir-a412ed]

Some local testing in the vagrant VM:

1. Make sure docker-init is present on the system.

# ls /usr/bin/docker-init -lt
-rwxr-xr-x 1 root root 804776 Oct  4 16:06 /usr/bin/docker-init

2. Without --init

job "count" {
  datacenters = ["dc1"]

  group "count-group" {
    count = 1
    task "count-task" {
      driver = "docker"
      config {
        image = "count:1.0"
      }

      resources {
        cpu    = 500
        memory = 256
      }
    }
  }
}

$ ./nomad job run test.nomad

# docker inspect -f {{.HostConfig.Init}} <container_id>
<nil>

3. With --init

job "count" {
  datacenters = ["dc1"]

  group "count-group" {
    count = 1
    task "count-task" {
      driver = "docker"
      config {
        image = "count:1.0"
        init      = true
      }

      resources {
        cpu    = 500
        memory = 256
      }
    }
  }
}

$ ./nomad job run test.nomad

# docker inspect -f {{.HostConfig.Init}} <containerd_id>
true

Website changes tested locally:

[schmichael]

Thanks @shishir-a412ed !

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