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 replaced URL for Debian 7.4 ISO with a correct one and added link to official Debian ISO sources.
Merge pull request #4279

a98b870 Some documentation fixes + link to my Docker/LXC guide (Giuseppe Mazzotta)