Add a Docker plugin

This just sets the `DOCKER_HOST` for OS X users so we don't have to keep
setting it all over the place.

plugins/available/docker.plugin.bash

@@ -0,0 +1,8 @@
+cite about-plugin
+about-plugin 'Helpers to get Docker setup correctly for boot2docker'
+
+# Note, this might need to be different if you have an older version
+# of boot2docker, or its configured for a different IP
+if [[ `uname -s` == "Darwin" ]]; then
+  export DOCKER_HOST=tcp://192.168.59.103:2375
+fi