Replace virtualbox with docker containers

[oxarbitrage]

By discussions in chat with @daira and @rex4539 in regards to https://github.com/zcash/zcash/issues/209 we are planning on making the gitian build process easier to consider it as our main build method in the future.

In order to do this we want to automate the steps to build zcash from sources with gitian. One of the things we want to do is replace virtualbox with containers. One option is docker, this issue is open to research and implement that.