Some documentation fixes + link to my Docker/LXC guide

I added a link to my guide about using docker containers + LXC (I am planning to maintain this at work for future bitcoin versions),
then I mentioned other virtualization options (KVM, LXC). This commit includes a fix issue for documentation issue #4269 that consists
in telling users to checkout correct bitcoin version before using the gitian descriptors (otherwise all hell can break loose).
Also, I provide correct URL for Debian 7.4 ISO and a link to official Debian ISO sources.