Support passing working directory (linked containers)

[clue]

This image currently exposes the directory /var/www. We should look into supporting Docker's workdir option so that this image can be used to browse any directory like this:

$ docker run -d --workdir=/boot/ -p 80:80 --link data:data clue/h5ai

In particular, this also supports browsing arbitrary folders from linked (data) containers:

$ docker run -v /data --name=data busybox true
$ docker run -d --workdir=/data -p 80:80 --link data:data clue/h5ai