starting docker tutorial

README

+We're going to be using docker in this course, though not as
+intensively as git. Still, it's worth taking some time to familiarize
+ourselves with it, especially since you're unlikely to be familiar
+with it.
+
+What is docker? You can think of it like a lightweight VM. It's
+really considerably different, because it uses the host processor,
+memory, network stack, etc., without creating virutal hardware. We
+can throw around terms like user-level filesystems, process groups,
+and network namespaces, but the important part is that you can run
+a self-contained guest Linux OS within another host Linux OS, with
+applications and all of their dependencies.  The guest can only see
+the resources given to it by the host, so it provides some (minimal)
+level of security. It also means we can start a process from a
+known-clean state, so we have repeatability.
+
+Exercise 1: Docker Images
+=========================
+
+Let's start with the concept of an *image*. This is the self-contained
+guest Linux OS, which is configured to automatically run some process
+when it starts.  Nothing is running in it -- you can think of it
+like a hard drive.
+
+The easiest way to get an image is to *pull* it from a *registry*. Docker
+has a default registry built in. Our VM is running Ubuntu 16.04, and it turns
+out there's an image available with this OS on it! Here's the command to run:
+
+    docker pull ubuntu:16.04
+
+Let's go through this command. "docker" is, of course, the utility
+we're using. The "pull" command tells us that we want to get something
+from a registry. In this case, we're getting the "ubuntu" image
+from the default registry. If we just left it at this, we'd get
+*all* of the ubuntu variants. Instead, we add ":16.04". That tells
+docker we only want one image, and it's the one with the *tag*
+"16.04".
+
+When the command completes, try running
+
+    docker image list
+
+You should see something like:
+
+    REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
+    ubuntu              16.04               2a4cca5ac898        28 hours ago        111MB
+
+Most of this should be fairly self-explanatory. The image ID is
+another hexadecimal number, like with git, but it's clearly not a
+SHA-1 hash. It really doesn't matter what it is, other than a unique
+identifier for this image.
+
+We can do a few things with this image, aside from running it. Try
+the following:
+
+    docker tag ubuntu:16.04 my_ubuntu
+    docker image list
+
+Note that we now see the same image ID twice, but with different
+names. By default, a repository (the tagless part of an image name)
+is tagged as "latest" if you don't specify one. Let's try specifying
+a tag, though:
+
+    docker tag ubuntu:16.04 foo:bar
+    docker image list
+
+The results should not be surprising.
+
+We can quickly build up a lot of images we don't want anymore, so
+it's good to know how to clean these up. Let's get rid of our new
+tagged images:
+
+    docker rmi my_ubuntu:latest foo:bar
+    docker image list
+
+A common problem is that we'll end up reusing an old tag, leaving
+an image with no repository:tag name. These show up as "<none>:<none>".
+We can get rid of all of these with the following bash one-liner:
+
+    docker images -a | grep none | awk '{print $3}' | xargs docker rmi
+
+For the curious, feel free to read the man pages for awk and xargs.
+This is not going to be essential information for this course,
+though.
+
+
+Exercise 2: Running an Image in a Container
+===========================================
+