Fix Dockerfile file copying best practices

Followed the usual advice of preferring COPY to ADD when adding files
into the Dockerfile, and made sure to only import the files needed for
the program to run.

Dockerfile

@@ -24,7 +24,7 @@ apt-get -q -y autoremove && apt-get -q -y clean && \
 rm -rf /var/lib/apt/lists/*
 
 # Final setup & execution
-ADD . /app
 WORKDIR /app
+COPY entrypoint.sh main.py /app
 ENTRYPOINT ["/bin/sh", "/app/entrypoint.sh"]
 CMD ["main.py"]