Changes included:
* Obtain the current dokuwiki released from the DOKUWIKI_VERSION
environment variable set in the Dockerfile.
* Only unpack the bundled dokuwiki release if we actually need to
copy over new files.
* Clean up the temporary copy of dokuwiki when we are done copying files
from it.
* Use `exec` to run the http server at the end of the startup script.
* Document the `donwgrade` and `overwrite` comamnds.
Miroslav Prasil
Andrei Poenaru