dockerfile: save rustup; rustup needs file
dockerfile: save rustup; rustup needs file

Signed-off-by: Tomas Tomecek <d6f442c575011cb3ced586e5014ff7dec58a6c29@redhat.com>

TomasTomecek/rust-container,TomasTomecek/rust-container,TomasTomecek/rust-container