Default registry username to Gitea actor

ci/README.md

@@ -35,15 +35,22 @@ REGISTRY_IMAGE=git.neosisyphus.com/evol3d/evol-testbed
 CI_IMAGE=git.neosisyphus.com/evol3d/evol-testbed
 ```
 
-Add these **Actions secrets**:
+Add this **Actions secret**:
 
 ```text
-REGISTRY_USERNAME=your-gitea-username
 REGISTRY_PASSWORD=your-gitea-token-or-password
 ```
 
 The token/password needs permission to publish packages/container images.
 
+Optional: add this secret if your registry username is different from the Gitea Actions actor:
+
+```text
+REGISTRY_USERNAME=your-gitea-username
+```
+
+If `REGISTRY_USERNAME` is not set, the workflow uses `$GITHUB_ACTOR`.
+
 ## Build manually
 
 ```bash