Update pip before resolving dependencies in container.

Author: tillahoffmann

Makefile

@@ -68,7 +68,7 @@ requirements : ${REQUIREMENTS}
 ${REQUIREMENTS} : requirements/%.txt : requirements.in */setup.py
 	mkdir -p $(dir $@)
 	${RUN} -w /workspace -v `pwd`:/workspace --platform=linux/amd64 python:$* bash -c \
-		"pip install pip-tools && pip-compile --resolver=backtracking -v --upgrade -o $@ $<"
+		"pip install --upgrade pip pip-tools && pip-compile --resolver=backtracking -v --upgrade -o $@ $<"
 
 # Remove any generated files.
 clean :