python: pin pip version to <19

pip version 19 has a bug[1] which is breaking our builds. Until it's
fixed, pin pip to a pre-19 version.

[1] pypa/pip#6158

python/Dockerfile

@@ -3,12 +3,15 @@ FROM python:$PYTHON_VERSION-alpine
 # Add our base requirements and install
 ADD ./requirements/ /tmp/requirements/
 
-# Update and upgrade system packages and install common Python modules
+# Update and upgrade system packages and install common Python modules.
+#
+# For the moment we pin pip at being less than 19.0 because of
+# https://github.com/pypa/pip/issues/6158.
 RUN apk --no-cache upgrade && \
 	apk --no-cache add git vim libffi libxml2 libxslt postgresql-client && \
 	apk --no-cache add \
 		postgresql-dev libffi-dev gcc g++ musl-dev libxml2-dev libxslt-dev libstdc++ && \
-	pip install --no-cache-dir --upgrade pip && \
+	pip install --no-cache-dir --upgrade 'pip<19.0' && \
 	pip install --no-cache-dir --upgrade --no-cache -r /tmp/requirements/$REQUIREMENTS.txt && \
 	apk del postgresql-dev libffi-dev gcc g++ musl-dev libxml2-dev libxslt-dev libstdc++