doc: clarify issue prefix conventions for container code (#2746)

Update CONTRIBUTING.md to document that container-related code lives at
`internal/container`, and issues for this area should use the lowercase
language name (e.g. `java:`) as a prefix for brevity.

For #2722

Author: julieqiu

CONTRIBUTING.md

@@ -34,7 +34,8 @@ can manually re-trigger it by:
 1. Clicking "View details" on the failed workflow to bring you to the "Actions" page.
 2. Clicking "Re-run failed jobs".
 
-For more information, see [Re-running failed jobs in a workflow](https://docs.github.com/en/actions/how-tos/managing-workflow-runs-and-deployments/managing-workflow-runs/re-running-workflows-and-jobs#re-running-failed-jobs-in-a-workflow).
+For more information, see
+[Re-running failed jobs in a workflow](https://docs.github.com/en/actions/how-tos/managing-workflow-runs-and-deployments/managing-workflow-runs/re-running-workflows-and-jobs#re-running-failed-jobs-in-a-workflow).
 
 ## Community Guidelines
 
@@ -107,8 +108,14 @@ accepted.
 
 All issues should have a path prefix to indicate the relevant domain. For
 issues related to the librarian codebase, use the package name as a prefix (for
-example, `librarian:` or `cli:`). For issues related to code outside this
+example, `librarian:` or `cli:`).
+
+For issues related to code outside this
 repository, use the repository name (for example, `google-cloud-python`).
+This repository also contains language-specific container-related code under
+[internal/container](https://github.com/googleapis/librarian/tree/main/internal/container).
+For these issues, use the lowercase language name as a prefix for brevity (for
+example, `java:`).
 
 Aside from proper nouns, issue titles should use lowercase.