Jenkins apache improvments proposal

Hi,

I would like to suggest the following improvements:
- apache2 fixes /etc/apache2/sites-available/jenkins.conf (conf was forgotten)
- apache2 fixes 000-default instead of default
- apache2 fixes default port must be changed to correspond to the vhosts
- plugins was not working with prefix (prefix has been added)
- create DockerFile in order to automatically create docker image from https://registry.hub.docker.com/ (this can be disabled)

Please do not hesitate to tell me, If you want me to improve this pull request.

Best regards,

Alban