test-in-docker: quickly test frameworks in docker

This adds `./test-in-docker` for quickly playing with various frameworks.

All the containers are based off Ubuntu 14.04 which has ZSH 5.0.2.

Thanks to @dritter for figuring out all the framework installation
methods.

TESTS.md

@@ -13,10 +13,23 @@ Tests in separate files under `test/functions`.
 These Tests tend to be more complex in setup than the basic tests. To avoid ending
 up in a huge single file, there is one file per segment in `test/segments`.
 
-# Test-VMs
+# Manual Testing
 
 If unit tests are not sufficient (e.g. you have an issue with your prompt that
-occurs only in a specific ZSH framework), then you could use our Test-VMs!
+occurs only in a specific ZSH framework) then you can use either Docker or
+or our Vagrant.
+
+## Docker
+
+This is the easiest to use _if_ you have Docker already installed and running.
+
+The command `./test-in-docker` should make it fairly easy to get into a running
+container with the framework of your choice.
+
+You can get Docker at <https://www.docker.com/community-edition>.
+
+## Vagrant
+
 Currently there are two test VMs. `test-vm` is an Ubuntu machine with several
 pre-installed ZSH frameworks. And there is `test-bsd-vm` which is a FreeBSD!
-For how to run the machines see [here](test-vm/README.md).
+For how to run the machines see [here](test-vm/README.md).