update Makefile to match new directory structure

Author: CaseyFaist

Makefile

@@ -21,7 +21,7 @@ test-heroku-18:
 buildenv-heroku-16:
 	@echo "Creating build environment (heroku-16)..."
 	@echo
-	@docker build --pull -t python-buildenv-heroku-16 .
+	@docker build --pull -f $(shell pwd)/builds/heroku-16.Dockerfile -t python-buildenv-heroku-16 .
 	@echo
 	@echo "Usage..."
 	@echo
@@ -34,7 +34,7 @@ buildenv-heroku-16:
 buildenv-heroku-18:
 	@echo "Creating build environment (heroku-18)..."
 	@echo
-	@docker build --pull -f Dockerfile.heroku-18 -t python-buildenv-heroku-18 .
+	@docker build --pull -f $(shell pwd)/builds/heroku-18.Dockerfile -t python-buildenv-heroku-18 .
 	@echo
 	@echo "Usage..."
 	@echo