doc: Detail configuration of hosted CI runners

Author: willcl-ark

ci/README.md

@@ -1,8 +1,8 @@
-## CI Scripts
+# CI Scripts
 
 This directory contains scripts for each build step in each build stage.
 
-### Running a Stage Locally
+## Running a Stage Locally
 
 Be aware that the tests will be built and run in-place, so please run at your own risk.
 If the repository is not a fresh git clone, you might have to clean files from previous builds or test runs first.
@@ -27,7 +27,7 @@ with a specific configuration,
 env -i HOME="$HOME" PATH="$PATH" USER="$USER" bash -c 'FILE_ENV="./ci/test/00_setup_env_arm.sh" ./ci/test_run_all.sh'
 ```
 
-### Configurations
+## Configurations
 
 The test files (`FILE_ENV`) are constructed to test a wide range of
 configurations, rather than a single pass/fail. This helps to catch build
@@ -49,8 +49,32 @@ env -i HOME="$HOME" PATH="$PATH" USER="$USER" bash -c 'MAKEJOBS="-j1" FILE_ENV="
 The files starting with `0n` (`n` greater than 0) are the scripts that are run
 in order.
 
-### Cache
+## Cache
 
 In order to avoid rebuilding all dependencies for each build, the binaries are
 cached and reused when possible. Changes in the dependency-generator will
 trigger cache-invalidation and rebuilds as necessary.
+
+## Configuring a repository for CI
+
+### Primary repository
+
+To configure the primary repository, follow these steps:
+
+1. Register with [Cirrus Runners](https://cirrus-runners.app/) and purchase runners.
+2. Install the Cirrus Runners GitHub app against the GitHub organization.
+3. Enable organisation-level runners to be used in public repositories:
+   1. `Org settings -> Actions -> Runner Groups -> Default -> Allow public repos`
+4. Permit the following actions to run:
+   1. cirruslabs/cache/restore@\*
+   1. cirruslabs/cache/save@\*
+   1. docker/setup-buildx-action@\*
+   1. actions/github-script@\*
+
+### Forked repositories
+
+When used in a fork the CI will run on GitHub's free hosted runners by default.
+In this case, due to GitHub's 10GB-per-repo cache size limitations caches will be frequently evicted and missed, but the workflows will run (slowly).
+
+It is also possible to use your own Cirrus Runners in your own fork with an appropriate patch to the `REPO_USE_CIRRUS_RUNNERS` variable in ../.github/workflows/ci.yml
+NB that Cirrus Runners only work at an organisation level, therefore in order to use your own Cirrus Runners, *the fork must be within your own organisation*.