Tweak pip args

Dockerfile

@@ -17,7 +17,7 @@ COPY requirements.txt /root/chasemapper/requirements.txt
 
 # Install Python packages.
 RUN pip3 --no-cache-dir install --user --no-warn-script-location \
-  --extra-index-url https://www.piwheels.org/simple \
+  --ignore-installed --no-binary numpy \
   -r /root/chasemapper/requirements.txt
 
 # Copy in chasemapper.