- fix Nix build on macOS Big Sur (by updating to nixpkgs-unstable)
- update gcc-arm-embedded from 9.x to 10.x
- update nix to 2.3.10
- update Alpine Linux to 3.12.3
- allow to change arch in Docker
Handling of full dependencies (multiple python versions, Monero tests)
was moved from Dockerfile to shell.nix.
The Python packages are installed from the pinned nixpkgs revision and
do not depend on channel state at the time of docker build anymore.
The Monero test binary is now downloaded using fetchurl which fails the
build if checksum doesn't match.
