Removed reference to ${PWD} from compose.yaml.

This isn't necessary - we can just use the current directory as there is
a lot of other stuff already doing that. Using `${PWD}` makes using this
on Windows more awkward as this environment variable is undefined by
default.

Author: ngnpope

compose.yml

@@ -4,7 +4,7 @@ x-base: &base
   image: django-docker-box:${PYTHON_IMPLEMENTATION}-${PYTHON_VERSION}
   build:
     context: .
-    dockerfile: ${PWD}/Containerfile
+    dockerfile: ./Containerfile
     args:
       - PYTHON_IMPLEMENTATION=${PYTHON_IMPLEMENTATION}
       - PYTHON_VERSION=${PYTHON_VERSION}