Explain why it was detected as self-hosted

common.js

@@ -179,6 +179,18 @@ export function isSelfHostedRunner() {
     getRunnerToolCache() !== getDefaultToolCachePath()
 }
 
+export function selfHostedRunnerReason() {
+  if (inputs.selfHosted === 'true') {
+    return 'the self-hosted input was set'
+  } else if (!GitHubHostedPlatforms.includes(getOSNameVersionArch())) {
+    return 'the platform does not match a GitHub-hosted runner image (or that image is deprecated and no longer supported)'
+  } else if (getRunnerToolCache() !== getDefaultToolCachePath()) {
+    return 'the $RUNNER_TOOL_CACHE is different than the default tool cache path (they must be the same to reuse prebuilt Ruby binaries)'
+  } else {
+    return 'unknown reason'
+  }
+}
+
 let virtualEnvironmentName = undefined
 
 export function getVirtualEnvironmentName() {

ruby-builder.js

@@ -28,8 +28,8 @@ export async function install(platform, engine, version) {
       if (common.isSelfHostedRunner()) {
         const rubyBuildDefinition = engine === 'ruby' ? version : `${engine}-${version}`
         core.error(
-          `The current runner (${common.getOSNameVersionArch()}, RUNNER_TOOL_CACHE=${common.getRunnerToolCache()}) was detected as self-hosted and not matching a GitHub-hosted runner image.\n` +
-          `In such a case, you should install Ruby in the $RUNNER_TOOL_CACHE yourself, for example using https://github.com/rbenv/ruby-build:\n` +
+          `The current runner (${common.getOSNameVersionArch()}, RUNNER_TOOL_CACHE=${common.getRunnerToolCache()}) was detected as self-hosted because ${common.selfHostedRunnerReason()}.\n` +
+          `In such a case, you should install Ruby in the $RUNNER_TOOL_CACHE yourself, for example using https://github.com/rbenv/ruby-build\n` +
           `You can take inspiration from this workflow for more details: https://github.com/ruby/ruby-builder/blob/master/.github/workflows/build.yml\n` +
           `$ ruby-build ${rubyBuildDefinition} ${toolCacheRubyPrefix}\n` +
           `Once that completes successfully, mark it as complete with:\n` +