Run the Docker container as a non-root user

[milancurcic]

The container currently runs code as a root user which provides possibility to break the running instance of the container, at least until the Python server process is restarted. Let's create a non-root user in the Docker image and use that to run code. Perhaps something like this:

ENV USERNAME=fortran
RUN useradd \
  --no-log-init \
  --home-dir /home/$USERNAME \
  --create-home \
  --shell /bin/bash \
  $USERNAME
USER $USERNAME
WORKDIR /home/$USERNAME

[milancurcic]

Fixed by #52