chore: add MODULE_DIRECTORY env var (#142)

This exists in the legacy template:

489ebea77b/build.sh (L13C1-L13C39)

And is also used in many modules, though with
`MODULE_DIRECTORY="${MODULE_DIRECTORY:-"/tmp/modules"}"` to not break
things.

And is documented on the website, because I assumed it existed:
https://blue-build.org/reference/module/#module_directory

I know this probably won't change much, but it's still useful for the
scripts.

template/templates/Containerfile.j2

@@ -14,6 +14,7 @@ ARG RECIPE={{ recipe_path.display() }}
 ARG IMAGE_REGISTRY={{ registry }}
 
 ARG CONFIG_DIRECTORY="/tmp/config"
+ARG MODULE_DIRECTORY="/tmp/modules"
 ARG IMAGE_NAME="{{ recipe.name }}"
 ARG BASE_IMAGE="{{ recipe.base_image }}"