dockerfiles: add a Dockerfile using a nightly Rust toolchain

dockerfiles: add a Dockerfile using a nightly Rust toolchain

This will be useful in order to test QEMU's Rust code with nightly
clippy.  It can also be used to check that the code builds, until the
minimum supported Rust version is lowered enough to allow enabling Rust
in other Dockerfiles too.

Use a separate container, instead of the Fedora one, to avoid that
CI breaks for everyone if for some reason the rustup build turns out
to be shaky.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>