Add a {{dir}} placeholder for relative commands

[frankcarey]

I want to reference something on the local file system in a docker exec command. If I was in the root folder (where .ahoy.yml file is) then I should be ok, but it would be nice to be able to do the same thing in a subdirectory. Ahoy could easily be smart enough to know the relative path of the directory ahoy is being called from and place that into {{dir}}

This is really useful for docker commands being run outside the container since you could know what the root is inside the container.