`--dir` hardcoded in entrypoint

[jeeftor]

In Dockerfile it looks like both --dir and --port are being "hardcoded.

These really should move to command ... so they can be overwritten. I'll make a PR :)

ENTRYPOINT ["./openbooks", "server", "--dir", "/books", "--port", "80"]

[jeeftor]

I actually have to validate this statement. If we can pass --port twice its a non-issue