Add /wiki/data volume

This change to the Dockerfile adds a volume for /wiki/data, which contains the changing data.
In a test I have use the following to make this work:

    # create data dir
    mkdir /home/<user>/wiki-data
    # build image
    docker build github.com/WardCunningham/Smallest-Federated-Wiki
    -> <image>
    # run container
    docker run -d -v /home/<user>/wiki-data:/wiki/data <image>
    -> <container>
    # find port
    docker port <container> 1111
    -> <port>
    # Visit http://localhost:<port>