message: Add docker installation message
diff: This example is a full end to end example of delivering a containerized Ruby on Rails application. It
  -* Provisions a [Google Kubernetes Engine (GKE)](https://cloud.google.com/kubernetes-engine/) cluster
  -* Provisions a fully managed Google Cloud SQL PostgreSQL database
  -* Builds a containerized Ruby on Rails container image, and publishes it to Docker Hub
  -* Deploys that container image as a Kubernetes Service inside of the provisioned GKE cluster
  +- Provisions a [Google Kubernetes Engine (GKE)](https://cloud.google.com/kubernetes-engine/) cluster
  +- Provisions a fully managed Google Cloud SQL PostgreSQL database
  +- Builds a containerized Ruby on Rails container image, and publishes it to Docker Hub
  +- Deploys that container image as a Kubernetes Service inside of the provisioned GKE cluster
  All of these happen behind a single `pulumi up` command, and are expressed in just a handful of TypeScript.
  ## Prerequisites
  Ensure you have [downloaded and installed the Pulumi CLI](https://www.pulumi.com/docs/get-started/install/).
  -
  +Esnure you have [downloaded and installed Docker](https://docs.docker.com/install/).
  We will be deploying to Google Cloud Platform (GCP), so you will need an account. If you don't have an account,
  [sign up for free here](https://cloud.google.com/free/). In either case,
  [follow the instructions here](https://www.pulumi.com/docs/intro/cloud-providers/gcp/setup/) to connect Pulumi to your GCP account.
  @@ -39,6 +39,7 @@ cluster and containerized Ruby on Rails application deployed into it, using a ho
  $ pulumi config set gcp:project [your-gcp-project-here]
  $ pulumi config set gcp:zone us-west1-a # any valid GCP zone works
  $ pulumi config set clusterPassword --secret [your-new-cluster-password-here]
  + $ pulumi config set dbUsername [your-new-db-username-here]
  $ pulumi config set dbPassword --secret [your-new-db-password-here]
  $ pulumi config set dockerUsername [your-dockerhub-username-here]
  $ pulumi config set dockerPassword --secret [your-dockerhub-password-here]
  