Add submodules in Dockerfile.

Dockerfile

@@ -1,9 +1,10 @@
-FROM        python:3.6
+FROM        python:3.7
 MAINTAINER  Michael B. Klein <michael.klein@northwestern.edu>
 RUN         mkdir -p /usr/local /data
 WORKDIR     /usr/local
 ADD         . lakesuperior
 WORKDIR     /usr/local/lakesuperior
+RUN         git submodule update --init
 RUN         pip install -e .
 RUN         cp ./docker/etc/* ./lakesuperior/etc.defaults/
 CMD         ./docker/docker_entrypoint