Fix example_app docker build, after moving to non-static versionning

Author: insspb

example_app/compose/flask/Dockerfile

@@ -1,6 +1,9 @@
 FROM python:3.7-slim as dev
 
 ENV PYTHONUNBUFFERED 1
+RUN apt-get update && \
+    apt-get install git -y --no-install-recommends && \
+    rm -rf /var/lib/apt/lists/*
 RUN groupadd -r flask && useradd -r -g flask flask
 COPY --chown=flask . /flask_mongoengine
 RUN pip install --upgrade pip  \