Retry image pushes on transient registry failures

The engine images carry a ~5.6GB torch layer. Uploading it intermittently
fails with a 502 from the reverse proxy in front of the registry, and the
manifest PUT that follows can fail with a 500 because the blob commit has
not registered yet. Both clear on a retry, but the script pushed each tag
exactly once, so a whole release — including a 25GB rebuild — could be lost
to one hiccup.

push_image() replaces the bare `run docker push` calls and retries up to
PUSH_RETRIES times (default 5) with PUSH_RETRY_DELAY seconds between
attempts (default 10). Exhausting the retries still returns non-zero so
set -e aborts the release rather than reporting a partial push as success.
Dry runs make no docker calls.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

DEVELOPER.md

@@ -101,6 +101,15 @@ Override the registry and org via environment variables:
 REGISTRY=ghcr.io IMAGE_ORG=myorg ./release-engine.sh --github
 ```
 
+Pushes are retried on transient registry failures. The engine images carry a
+~5.6GB torch layer, and uploading it can fail with a 502 from the proxy in
+front of the registry (or a 500 on the manifest PUT that follows), which
+clears on a retry. Tune with:
+
+```bash
+PUSH_RETRIES=8 PUSH_RETRY_DELAY=20 ./release-engine.sh --gitea
+```
+
 ## API reference
 
 All endpoints are under `/api/v1/`. Requires `Authorization: Bearer <key>` header when `KB_API_KEY` is set.