Some flaws in rust-toolchain.toml and Dockerfile

[jklincn]

1. rust-toolchain.toml

   • It would be better to modify llvm-tools-preview to llvm-tools.
   • More evidence:
     • rustup-components-history
     • fix(rust-toolchain): Fix schema for rust-toolchain.toml

2. Dockerfile

   • There is a flaw of a duplicate software package installation, such as git and curl.
   • It can use multi-stage building to reduce image size. For example, move the compilation of QEMU to another build stage.
   • Since the docker image is built up from a series of layers, RUN rm foo after RUN wget foo could not reduce image size.
   • Due to the special network environment, should a mirror or accelerated source be configured?