no need for seperate numpy step. matplotlib brings it in

Dockerfile

@@ -24,14 +24,13 @@ RUN docker-apt-install \
     #pkg-config \
     #python-dev \
 
-# install deps that don't depend on the code as the user
-RUN mkdir /src \
- && chown abc:abc /src \
- && chroot --userspec=abc / pip install matplotlib numpy
-# todo: do something to calculate font cache with matplotlib
-#       /home/joinmarket/pyenv/local/lib/python2.7/site-packages/matplotlib/font_manager.py:273: UserWarning: Matplotlib is building the font cache using fc-list. This may take a moment.
-
-# copy requirements before code. this will make the image builds faster when developing
+# install deps that don't depend on the code as the user and fix /pyenv/local/lib/python2.7/site-packages/matplotlib/font_manager.py:273: UserWarning: Matplotlib is building the font cache using fc-list. This may take a moment.
+# TODO: double check that this actually builds the font caches
+RUN chroot --userspec=abc / pip install matplotlib==2.0.0 \
+ && chroot --userspec=abc / python -c \
+    "from matplotlib.font_manager import FontManager; print(FontManager())"
+
+# copy requirements before code. this will make the image builds faster when code changes but requirements don't
 COPY requirements.txt /src/
 RUN chroot --userspec=abc / pip install -r /src/requirements.txt