Fix Dockerfile

[dashohoxha]

https://github.com/docker-scripts/privoxy/blob/master/Dockerfile

Try to make it similar to squid: https://github.com/docker-scripts/squid/blob/master/Dockerfile

Any docker-scripts application should have this line in Dockerfile: CMD ["/sbin/init"]
Otherwise, probably it should not be built with docker-scripts.

@docker-scripts/ds