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dockerfile: add a rust toolchain for wasm builds

A repository that compiles to WebAssembly could not be built in the box
at all. rustup rather than Debian's rustc, because bookworm ships 1.63
and a crate declaring edition = "2024" needs 1.85 or newer, so
apt install rustc yields a toolchain that cannot so much as parse the
manifest.

cargo-run-bin comes with it. Projects that pin their build tools
through [package.metadata.bin] -- wasm-bindgen-cli, wasm-opt -- invoke
cargo bin from their own build scripts, so a toolchain lacking that
subcommand still fails at the first step. It is installed with
--root /usr/local, which puts the binary on the default PATH rather
than in a CARGO_HOME thrown away later in the same layer.

RUSTUP_HOME is baked into /opt and left root-owned, which is safe
because the shims only read it. CARGO_HOME is deliberately not set, so
it falls back to $HOME/.cargo: writable by the unprivileged runtime
user, and mountable with rw = ["~/.cargo"] to keep the registry and
git checkouts across boxes. Left alone that directory is part of the
box and goes away with it, so a Rust build starts from an empty cache
each time.

Signed-off-by: Andrew Udvare <audvare@gmail.com>

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