Instead of hardcoding the version of the Rust alpine file, introduce an `ARG` in
the `Dockerfile`. In `build/build`, we determine the version by inspecting
`rust-toolchain.toml`, and pass this value in. This ensures that the binaries
are built using the same toolchain that we develop with.
`--env` is a podman-specific feature. While it is clear that podman is a
requirement here, switching these to build arguments better expresses
intent.
These values are meant to be configurable at build time, which is
exactly what build arguments are for, while environment variables are
meant to persist in the image built.
Signed-off-by: Yorgos Saslis <yorgos.work@proton.me>