Upgrade pip also.

Dockerfile

@@ -108,7 +108,7 @@ RUN \
     \
     # OS dependencies
     apt-get update -y && apt-get install -y --no-install-recommends ${BUILD_DEPS} && \
-    pip3 install -U setuptools wheel && \
+    pip3 install -U pip setuptools wheel && \
     \
     # Meteor installer doesn't work with the default tar binary, so using bsdtar while installing.
     # https://github.com/coreos/bugs/issues/1095#issuecomment-350574389