BUG: ImportError in system pip wrappers after an upgrade

third_party/caffe/Dockerfile

@@ -31,8 +31,10 @@ WORKDIR $CAFFE_ROOT
 # https://github.com/docker/hub-feedback/issues/460
 ENV CLONE_TAG=1.0
 
+# ImportError in system pip wrappers after an upgrade.
+# https://github.com/pypa/pip/issues/5599
 RUN git clone -b ${CLONE_TAG} --depth 1 https://github.com/BVLC/caffe.git . && \
-    pip install --upgrade pip && \
+    python -m pip install --upgrade pip && \
     cd python && for req in $(cat requirements.txt) pydot; do pip install $req; done && cd .. && \
     mkdir build && cd build && \
     cmake -DCPU_ONLY=1 .. && \