Set default configuration in Dockerfile using environment variables

[AlexandreCarlton]

This will allow users to easily change the port and directory without having to override the entrypoint.

I also updated the README to document the usage of these environment variables.

[nicolov]

LGTM, or we could change ENTRYPOINT to CMD IIRC.

[AlexandreCarlton]

I considered this, but if we had:

CMD ["--port=80", "--dir=/data"]

then a user already providing arguments to the docker run invocation (for example --max-size 1) would override these options instead of appending to them. From the docs:

If the user specifies arguments to docker run then they will override the default specified in CMD.

Specifying these values as environment variables seemed better to avoid breaking changes.