Added openxpkidownload volume to openxpki-client and openxpki-server

[rxt30]

Currently CRLs and CA-certificates could not be retrieved by clients without the user interface, as the files were generated to /var/www/download on the openxpki-server container, and the webserver in the openxpki-client container serves it's own /var/www/download, without the generated files in it.

This pull request mounts /var/www/download/ into a volume (openxpkidownload) in both containers, so that they have the same content in /var/www/download/.

[maxi322]

I had the same problem and implemented the same solution into my docker compose environment some time ago. This should definitely be added.

[rriemann]

Please consider this for inclusion to master