The Rust upgrade is necessary since our Cargo.lock file now requires
`ed25519` 1.4.0, which requires edition2021, which requires Rust
1.56 or later.
The Alpine upgrade is opportunistic: we might as well.
I've also added comments to remind us to keep the .gitlab-ci.yml
and docker_reproducible_build files in sync, since my first version
of this commit messed that up.
Closes#376.
As per #309
Update all references. There is one remaining hit for
git-grep '\.sh\b'
namely "build.sh" in maint/reproducible_build, which I think is
the build script for osxcross - ie, an external project.