docs: note that backend pybabel compile errors are swallowed

The python-translation-compiler stage runs pybabel compile with
`|| true`, so a broken .po file won't fail the build. Point that
out next to the BUILD_TRANSLATIONS ARG doc so it's not a surprise.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/admin_docs/installation/docker-builds.mdx

@@ -164,7 +164,9 @@ came out of, credit to the community for working it out.
   steps are skipped and only `en` ships. This only takes effect when building the image from source
   (`docker build` against this repo's own `Dockerfile`); it has no effect on a downstream
   Dockerfile that just extends an already-published tag, see
-  "Adding translations to a custom image" above.
+  "Adding translations to a custom image" above. Note that the backend `pybabel compile`
+  step ignores its exit code, so a `.po` file with a compile error won't fail the build;
+  check the build logs for `pybabel` warnings if a locale's backend strings aren't showing up.
 - `DEV_MODE`: whether to skip the frontend build, this is used by our `docker-compose` dev setup
   where we mount the local volume and build using `webpack` in `--watch` mode, meaning as you
   alter the code in the local file system, webpack, from within a docker image used for this