Update Dockerfile

For security reasons, run the Docker container as an unprivileged user

Dockerfile

@@ -1,5 +1,12 @@
 FROM python:3.7-slim
 ADD . /code
 WORKDIR /code
-RUN pip install -r requirements.txt
+RUN \
+  groupadd -r webssh && \
+  useradd -r -s /bin/false -g webssh webssh && \
+  chown -R webssh:webssh /code && \
+  pip install -r requirements.txt
+
+EXPOSE 8888/tcp
+USER webssh
 CMD ["python", "run.py"]