Create volume mount point for data in Dockerfile (#104)

`/opt/opendj/data` does not exist in the image. Because of this, any
volume mounted to that location will be owned by `root` and unwritable
by `$OPENDJ_USER` by default.

By creating this directory (owned by `$OPENDJ_USER`), a newly created
volume mounted on `/opt/opendj/data` will get its permissions set to
whatever the underlying directory is set to, so OpenDJ can write to it.