Install `git` on pip image (#443)

Needed for DuniterPy installation from git

image/pip

@@ -9,6 +9,11 @@ WORKDIR /silkaj
 # Copy source tree
 COPY ./ ./
 
+# Install git
+RUN apt update && \
+    apt install --yes git && \
+    rm -rf /var/lib/apt/lists
+
 # Install Silkaj
 RUN pip install . silkaj[deathreaper]