Reword submodule handling

Author: Kobzol

Diff for: src/tests/docker.md

@@ -21,7 +21,7 @@ The [`src/ci/docker/run.sh`] script is used to build a specific Docker image, ru
 build Rust within the image, and either run tests or prepare a set of archives designed for distribution. The script will mount your local Rust source tree in read-only mode, and an `obj` directory in read-write mode. All the compiler artifacts will be stored in the `obj` directory. The shell will start out in the `obj`directory. From there, it will execute `../src/ci/run.sh` which starts the build as defined by the Docker image.
 
 You can run `src/ci/docker/run.sh <image-name>` directly. A few important notes regarding the `run.sh` script:
-- There is some configuration used on CI that you may need to recreate. In particular, set `submodules = false` in your `config.toml` so that it doesn't attempt to modify the read-only directory.
+- When executed on CI, the script expects that all submodules are checked out. If some submodule that is accessed by the job is not available, the build will result in an error. You should thus make sure that you have all required submodules checked out locally. You can either do that manually through git, or set `submodules = true` in your `config.toml` and run a command such as `x build` to let bootstrap download the most important submodules (this might not be enough for the given CI job that you are trying to execute though).
 - `<image-name>` corresponds to a single directory located in one of the `src/ci/docker/host-*` directories. Note that image name does not necessarily correspond to a job name, as some jobs execute the same image, but with different environment variables or Docker build arguments (this is a part of the complexity that makes it difficult to run CI jobs locally).
 - If you are executing a "dist" job (job beginning with `dist-`), you should set the `DEPLOY=1` environment variable.
 - If you are executing an "alternative dist" job (job beginning with `dist-` and ending with `-alt`), you should set the `DEPLOY_ALT=1` environment variable.