Update docs on using Docker images.

Author: ehuss

src/tests/intro.md

@@ -136,16 +136,54 @@ Rust's [platform tiers]).
 ## Testing with Docker images
 
 The Rust tree includes [Docker] image definitions for the platforms used on
-Azure Pipelines in [src/ci/docker].  The script [src/ci/docker/run.sh] is used to build
+Azure Pipelines in [`src/ci/docker`].  The script [`src/ci/docker/run.sh`] is used to build
 the Docker image, run it, build Rust within the image, and run the tests.
 
-> TODO: What is a typical workflow for testing/debugging on a platform that
-> you don't have easy access to?  Do people build Docker images and enter them
-> to test things out?
+You can run these images on your local development machine. This can be
+helpful to test environments different from your local system. First you will
+need to install Docker on a Linux, Windows, or macOS system (typically Linux
+will be much faster than Windows or macOS because the later use virtual
+machines to emulate a Linux environment). To enter interactive mode which will
+start a bash shell in the container, run `src/ci/docker/run.sh --dev <IMAGE>`
+where `<IMAGE>` is one of the directory names in `src/ci/docker` (for example
+`x86_64-gnu` is a fairly standard Ubuntu environment).
+
+The docker script will mount your local rust source tree in read-only mode,
+and an `obj` directory in read-write mode. All of the compiler artifacts will
+be stored in the `obj` directory. The shell will start out in the `obj`
+directory. From there, you can run `../src/ci/run.sh` which will run the build
+as defined by the image.
+
+Alternatively, you can run individual commands to do specific tasks. For
+example, you can run `python3 ../x.py test src/test/ui` to just run UI tests.
+Note that there is some configuration in the [`src/ci/run.sh`] script that you
+may need to recreate. Particularly, set `submodules = false` in your
+`config.toml` so that it doesn't attempt to modify the read-only directory.
+
+Some additional notes about using the Docker images:
+
+- Some of the std tests require IPv6 support. Docker on Linux seems to have it
+  disabled by default. Run the commands in [`enable-docker-ipv6.sh`] to enable
+  IPv6 before creating the container. This only needs to be done once.
+- The container will be deleted automatically when you exit the shell, however
+  the build artifacts persist in the `obj` directory. If you are switching
+  between different Docker images, the artifacts from previous environments
+  stored in the `obj` directory may confuse the build system. Sometimes you
+  will need to delete parts or all of the `obj` directory before building
+  inside the container.
+- The container is bare-bones, with only a minimal set of packages. You may
+  want to install some things like `apt install less vim`.
+- You can open multiple shells in the container. First you need the container
+  name (a short hash), which is displayed in the shell prompt, or you can run
+  `docker container ls` outside of the container to list the available
+  containers. With the container name, run `docker exec -it <CONTAINER>
+  /bin/bash` where `<CONTAINER>` is the container name like `4ba195e95cef`.
 
 [Docker]: https://www.docker.com/
-[src/ci/docker]: https://github.com/rust-lang/rust/tree/master/src/ci/docker
-[src/ci/docker/run.sh]: https://github.com/rust-lang/rust/blob/master/src/ci/docker/run.sh
+[`src/ci/docker`]: https://github.com/rust-lang/rust/tree/master/src/ci/docker
+[`src/ci/docker/run.sh`]: https://github.com/rust-lang/rust/blob/master/src/ci/docker/run.sh
+[`src/ci/run.sh`]: https://github.com/rust-lang/rust/blob/master/src/ci/run.sh
+[`enable-docker-ipv6.sh`]: https://github.com/rust-lang/rust/blob/master/src/ci/scripts/enable-docker-ipv6.sh
 
 ## Running tests on a remote machine