FIX:Makefile: add PWD environment variable

The previous makefile assumed that PWD was populated. This doesn't
happen in certain cases (e.g., when running as a systemd unit). Make
sure we have the PWD variable set before running any target.

Makefile

@@ -2,6 +2,7 @@ DOCKER_USER:=pierres
 DOCKER_ORGANIZATION=archlinux
 DOCKER_IMAGE:=base
 BUILDDIR=build
+PWD=$(shell pwd)
 
 hooks:
 	mkdir -p alpm-hooks/usr/share/libalpm/hooks