ci: reuse published images when the image sources are unchanged

Release hashes services/, migrations/ and config/; when images for that hash
already exist it skips CI and the build and only applies the new tags. Tagging
a release no longer recompiles the webmail.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -87,7 +87,9 @@ remain are listed in the README under
   `git.lucasrguerra.dev.br/lucasrguerra/lambdamail/<service>` and the `v*` tag
   creates a release with notes. See [docs/RELEASING.md](docs/RELEASING.md).
 - CI caches npm, Go modules, the embedded Postgres, Trivy's database and image
-  layers.
+  layers. Release recognises images it already published for the same sources
+  (`inputs-<hash>` tag), so a documentation change or a version tag never
+  rebuilds them.
 - One `.env.example` for development and production, and `scripts/init-env.sh`
   to create `.env` with random secrets (`make env`). It replaces
   `.env.production.example`.