Reduced the docker image size

docker/Dockerfile

 FROM python:3.6-stretch
 ARG SDIST
-RUN mkdir /app/
-COPY $SDIST /app/
-COPY requirements.txt /app/
-COPY tests/ /app/tests/
-WORKDIR /app/
+COPY $SDIST .
 RUN apt-get update \
 	&& apt-get install -y --no-install-recommends \
-		r-bioc-limma
-RUN pip3 install wheel
-RUN pip3 install fractalis-*.tar.gz gunicorn
+	    r-bioc-limma \
+    && pip3 install wheel gunicorn \
+    && pip3 install fractalis-*.tar.gz \
+    && rm -rf /var/lib/apt/lists/*