enable to cache cargo's registry and build in gitlab ci

[jm-observer]

if somebody need, it is my ci's script that can enable to cache.

1. because ci's cache only can use paths that are in the local working copy, so we must mkdir a folder in our project for cargo's registry (it is "target/registry" in my script), and it is my project structure:

   .
   ├── Cargo.toml
   ├── src
   │       ├── main.rs
   ├── target
   │   └── registry

2. the script:

   build-job:
   stage: build
   only:
   refs:
     - master
   tags:
   - maven
   image: messense/rust-musl-cross:x86_64-musl
   script:
   - ln -s $(pwd)/target/registry /root/.cargo/registry
   - export CARGO_BUILD_DEP_INFO_BASEDIR=.
   - cargo build --package endpoint --bin endpoint --release --target=x86_64-unknown-linux-musl
   cache:
   key: moss-endpoint-cache
   paths:
     # cache for cargo registry
     - target/registry/
     # cache for build
     - target/x86_64-unknown-linux-musl/
     - target/release/