A few documentation fixes + added link to my Docker/LXC gitian-builder guide

* provide correct URL for Debian 7.4 ISO + link at official ISO sources (was mentioned in this comment: https://github.com/bitcoin/bitcoin/issues/4265#issuecomment-44834620)
* fix issue #4269: tell user to checkout correct bitcoin version before using the gitian descriptors
* add link to [my guide + templates](https://github.com/gdm85/tenku/blob/master/docker/gitian-bitcoin-host/README.md) about using docker containers + LXC (I am planning to maintain this at work, also for future bitcoin versions)
* temporary workaround for [gitian-builder issue with environment](https://github.com/devrandom/gitian-builder/issues/56)