vscode: add the "clean", "unit test" and "code sytle check" tasks in the file: "./vscode/tasks.json" and improve the corresponding help document file: "docs/howto/how_to_build_and_release.md". A small bug is also fixed in the "start_build_docker" function of the file "apollo_docker.sh" when using vscode to pull the docker image from China.   (#3309)

* vscode: add the "clean" and "code sytle check" tasks in the file: "./vscode/tasks.json" and improve the corresponding help document file: "docs/howto/how_to_build_and_release.md". A small bug is also fixed in the "start_build_docker" function of the file "apollo_docker.sh" when using vscode to pull the docker image from China.