Make small Alpine images

[palfrey]

We'll need to use something like https://hub.docker.com/r/ekidd/rust-musl-builder/ as there isn't proper rustup support for Musl/Alpine yet

[palfrey]

See rust-lang/rustup#354

[palfrey]

rust-lang/rust#57439 is working towards this

[palfrey]

This would work, but we need Rust 1.32, and Alpine currently only has 1.31 (see https://pkgs.alpinelinux.org/package/edge/community/x86_64/rust)

[palfrey]

This is possibly doable on other platforms. See https://medium.com/@rajasekar3eg/rust-for-python-developers-environment-setup-6752315a61bd for how to build small static images