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.
The patch to make it possible to configure the LXC IPs has been merged
upstream. This means that it is no longer needed to patch gitian.
Remove that workaround.
