Reproducible builds of unit-e

[cornelius]

Bitcoin uses gitian to create reproducible builds, i.e. deterministic builds which create the same binary result even when run by different people and at different times. They use this to create the Bitcoin Core release builds. Signatures of builds by different people are collected in the gitian.sigs and once they have signatures on matching results from at least three different people, they publish the release.

We need to look into this system, check if it works with the current unit-e code and what would have to be changed to make it work, and decide if we want to use it for our release builds as well.

The advantage would be that we can leverage what upstream does and have a decentralized system to validate release builds which doesn't require something like a trusted build server.

The disadvantage is that the system is quite complex, goes through several layers of abstractions, only works in a narrowly defined environment (which can be run in a virtual machine, though), and takes some time to result in published release builds.

[Ruteri]

#511 adapts gitian build script and descriptors to Unit-e. It is now possible to build Unit-e and extract the signatures.

[cornelius]

Reproducible builds of unit-e with gitian are functional now. We cherry-picked the latest upstream version of the gitian builds (#508, #548) and made it work with unit-e (#511). There also are quite a bit of improvements to the build script (e.g. #549, #550, #551). This is a solid working base now.

We still need to put an actual release through the gitian builds and make sure that the builds and signing and verifying releases works. We will do that with a first 0.0.1 release as soon as we have a first working version of what we intend to release in master.

We have removed the Qt based wallet now (#689, #676, #569) and also don't build the DMGs for Mac anymore (#663). This simplifies the build system quite a bit.

The gitian build script now also works on Mac (#610), building in a Docker container.

As the general work is done now, I'll close this issue. We'll create more specific follow-up issues as needed.