chore(Dockerfile): update pip to 8.1.2

See https://pip.pypa.io/en/stable/news/#release-notes

Author: mboersma

rootfs/Dockerfile

@@ -12,7 +12,7 @@ RUN apt-get update \
 		python \
 		python-dev \
 		--no-install-recommends \
-	&& curl -sSL https://bootstrap.pypa.io/get-pip.py | python - pip==8.1.1 \
+	&& curl -sSL https://bootstrap.pypa.io/get-pip.py | python - pip==8.1.2 \
 	&& pip install --disable-pip-version-check --no-cache-dir docker-py==1.8.1 \
 	&& apt-get remove -y --auto-remove --purge \
 		gcc \