For Azure: Fix rate limiting issue for pulling docker image

[Rykarix]

According to various sources, docker recently changed their rate limiting for anonymous sources.

The following changes will allow users to deploy to azure without failing:

In variables.tf, add the following:

variable "docker_password" {
  description = "Docker Hub Password"
  type        = string
  sensitive   = true
}

variable "docker_username" {
  description = "Docker Hub Username"
  type        = string
  sensitive   = true
}

In main.tf, place this after line 74:

  image_registry_credential {
    server   = "index.docker.io"
    password = var.docker_password
    username = var.docker_username
  }

The user will then have 2 options. Either supply the credentials using export, or just run apply and the console will request the user details