Fix nil pointer dereference in auth config

[izeau]

Hello!

I noticed some nil pointer dereference in the deploy stage using this (basic) configuration:

project = "pet-shop"

app "api" {
  build {
    use "docker" {}

    registry {
      use "docker" {
        insecure = true
        image    = "registry.waypoint:5000/pet-shop-api"
        tag      = gitrefpretty()
      }
    }
  }

  deploy {
    use "docker" {
      service_port = 80
    }
  }
}

It looks like the pullImage function in builtin/docker/platform.go checks if the auth block is empty or not, but fails to check for nil. After taking a quick look around, I found this bug in 3 more cases. I applied the same reasoning as @catsby in e5b5fe9 to fix them.