Replace virtualbox with docker containers

[oxarbitrage]

By discussions in chat with @daira and @rex4539 in regards to zcash/zcash#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.

[rex4539]

Just make sure that whatever method you use, it's still possible to compile directly from macOS and it's not strictly required to use a Linux box.

[mdr0id]

Upstream appears to have made this all configurable during their bash to python infra work:

https://github.com/bitcoin/bitcoin/blob/master/contrib/gitian-build.py

We are bringing in some of the scaffolding infrastructure to help the pivot away from vbox and lxc components.

See: https://hub.docker.com/r/electriccoinco

[oxarbitrage]

With permission of the initial author i forked a little MIT licensed project to help with gitian builds. Basically is wrapper and dont expect to replace this repo but just an additional tool to have.

https://github.com/oxarbitrage/zcash-gitian

This use the same descriptors as we have in https://github.com/zcash/zcash/tree/master/contrib/gitian-descriptors

The linux version is the only one expected to work (others are downloading the bitcoin repo: https://github.com/zcash/zcash/blob/master/contrib/gitian-descriptors/gitian-osx.yml#L24 , https://github.com/zcash/zcash/blob/master/contrib/gitian-descriptors/gitian-win.yml#L24 and they also have further issues) but eventually i think we can make them all work.

Tests made from linux(mint-ubuntu18.04), building linux(debian jessie) and with docker. If there is some interest in the project will be great to continue the development of it.