Add a `healthcheck.sh` script

Useful for checking if the application is healthy

exec wget --spider --quiet 'http://127.0.0.1:8000'

Add this to the dockerfile:

#
# COPY --from=build /blog/build/erlang-shipment /blog
COPY healthcheck.sh /blog/healthcheck.sh

RUN chmod +x /blog/healthcheck.sh
HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 CMD [ "/blog/healthcheck.sh" ]