Introduce the docker-bash tool.

This is a shortcut tool for logging into a container using SSH. Usage: `docker-bash <CONTAINER_ID>`.

README.md

@@ -50,6 +50,7 @@ You can configure the stock `ubuntu` image yourself from your Dockerfile, so why
      * [Using the insecure key for one container only](#using_the_insecure_key_for_one_container_only)
      * [Enabling the insecure key permanently](#enabling_the_insecure_key_permanently)
      * [Using your own key](#using_your_own_key)
+     * [The `docker-bash` tool](#docker_bash)
    * [Disabling SSH](#disabling_ssh)
  * [Building the image yourself](#building)
  * [Conclusion](#conclusion)
@@ -358,6 +359,27 @@ Now SSH into the container as follows:
 
     ssh -i /path-to/your_key root@<IP address>
 
+<a name="docker_attach"></a>
+#### The `docker-bash` tool
+
+Looking up the IP of a container and running an SSH command quickly becomes tedious. Luckily, we provide the `docker-bash` tool which automates this process. This tool is to be run on the *Docker host*, not inside a Docker container.
+
+First, install the tool on the Docker host:
+
+    curl --fail -L -O https://github.com/phusion/baseimage-docker/archive/master.tar.gz && \
+    tar xzf master.tar.gz && \
+    sudo ./baseimage-docker-master/install-tools.sh
+
+Then run the tool as follows to login to a container using SSH:
+
+    docker-bash YOUR-CONTAINER-ID
+
+You can lookup `YOUR-CONTAINER-ID` by running `docker ps`.
+
+By default, `docker-bash` will open a Bash session. You can also tell it to run a command, and then exit:
+
+    docker-bash YOUR-CONTAINER-ID echo hello world
+
 
 <a name="building"></a>
 ## Building the image yourself