Allow publishers to retain every remote image tag

Fixes bug-bkhzrg4dzz00

Authored-By: Codex (GPT-6) <noreply@openai.com>

README.md

@@ -31,6 +31,7 @@ local Docker daemon, image-push uploads it.
 | `org` | yes | — | Gitea org for package API (`fritzlab`, `dns`). |
 | `name` | yes | — | Package name as registered in the registry. |
 | `latest` | no | `true` | Also push a `:latest` tag. |
+| `prune` | no | `true` | Prune old numeric registry tags. Set `false` for retained or independently pinned artifacts. |
 | `keep` | no | `3` | Numeric tags to retain. Older are deleted. |
 
 ## Behavior
@@ -38,6 +39,16 @@ local Docker daemon, image-push uploads it.
 1. `docker login code.fritzlab.net` as `ci-bot`.
 2. `docker push <image>:<tag>`.
 3. If `latest=true`, also `docker push <image>:latest`.
-4. Prune: list numeric tags from Gitea package API, keep the newest `keep`,
+4. If `prune=true`, list numeric tags from Gitea package API, keep the newest `keep`,
    delete the rest. Failures here do not fail the workflow
    (`continue-on-error: true`).
+
+`prune` accepts only the strings `true` and `false`; invalid input fails before
+registry login or push. With `false`, remote package listing and deletion are
+skipped. Image upload and local Docker cleanup remain unchanged. The action does
+not discover deployment pins; owners choosing pruning must account for that
+retention contract.
+
+Run `python3 -m unittest discover -s tests` with the pinned test dependencies in
+`tests/requirements.txt`. Tests inspect the composite action and execute only
+its credential-free validation step; no registry or package deletion runs.