Docker - make sure scheduler is loaded as submodule

docker/Dockerfile

@@ -2,13 +2,15 @@ FROM python:2.7-slim
 
 RUN apt-get update -y \
     && mkdir -p /usr/share/man/man1 /usr/share/man/man7 \
-    && apt-get install -y --no-install-recommends postgresql-client \
+    && apt-get install -y --no-install-recommends postgresql-client git-core \
     && rm -rf /var/lib/apt/lists/* \
     && apt-get clean \
     ;
 
-ARG REQUIREMENT=https://github.com/dalibo/temboard/archive/master.zip
-RUN pip install --no-cache-dir $REQUIREMENT
+RUN git clone --depth=1 --recursive https://github.com/dalibo/temboard/ && \
+    pip install --no-cache-dir ./temboard && \
+    rm -rf temboard
+
 RUN python -c 'import urllib; urllib.urlretrieve("https://github.com/tianon/gosu/releases/download/1.10/gosu-amd64", "/usr/local/bin/gosu")' \
     && chmod 0755 /usr/local/bin/gosu \
     && python -c 'import urllib; urllib.urlretrieve("https://raw.githubusercontent.com/vishnubob/wait-for-it/master/wait-for-it.sh", "/usr/local/bin/wait-for-it")' \