Update updating-docker docs

Author: keithduncan

docs/updating-docker.md

@@ -1,15 +1,23 @@
-# Updating Docker
+# Updating Docker and docker-compose
+
+Docker and docker-compose are built into the AMIs by the Packer build.
 
 See https://github.com/buildkite/elastic-ci-stack-for-aws/pull/954 for an
-example of updating Docker for Linux and Windows.
+example of updating Docker and docker-compose for Linux and Windows.
+
+1. Create a new branch
+1. Update and commit a change to the Packer install scripts for [Linux](#linux) and [Windows](#windows) for the new version(s)
+1. Push your branch and open a pull request
+1. Wait for CI to go green
+1. Merge
 
 ## Linux
 
 Packer installs Docker, docker-compose, and `docker buildx` using the [`packer/linux/scripts/install-docker.sh`](packer/linux/scripts/install-docker.sh)
 script. Update the `DOCKER_VERSION` variable in this file to change which
 version of Docker is installed. Update the `DOCKER_COMPOSE_VERSION` variable to
 change which version of docker-compose is installed. Update the
-`DOCKER_BUILDX_VERSION`variable to change which version of `docker buildx` is installed.
+`DOCKER_BUILDX_VERSION` variable to change which version of `docker buildx` is installed.
 
 Binary releases are downloaded directly from Docker and GitHub.