Default registry username to Gitea actor

ci/USE_CI_IMAGE.md

@@ -169,24 +169,28 @@ In your repository:
 1. Go to **Settings**.
 2. Go to **Actions**.
 3. Go to **Secrets**.
-4. Add:
+4. Add this required secret:
 
 ```text
-REGISTRY_USERNAME
 REGISTRY_PASSWORD
 ```
 
 Example:
 
 ```text
-REGISTRY_USERNAME=myusername
 REGISTRY_PASSWORD=<your-gitea-access-token>
 ```
 
-Use your Gitea username for `REGISTRY_USERNAME`.
-
 Use the token from step 3 for `REGISTRY_PASSWORD`.
 
+Optional: add this secret if your registry username is different from the Gitea Actions actor:
+
+```text
+REGISTRY_USERNAME=myusername
+```
+
+If `REGISTRY_USERNAME` is not set, the workflow uses `$GITHUB_ACTOR`.
+
 ---
 
 ## 6. Check the image build workflow
@@ -592,13 +596,10 @@ This avoids surprise breakage when `clang22` or `latest` changes.
 ```text
 1. Commit ci/Dockerfile and workflows.
 2. Create Gitea token with package/container write access.
-3. Add Actions variables:
-   - REGISTRY_HOST
-   - REGISTRY_IMAGE
-   - CI_IMAGE
+3. Registry/image values are already hardcoded in the workflow.
 4. Add Actions secrets:
-   - REGISTRY_USERNAME
    - REGISTRY_PASSWORD
+   - REGISTRY_USERNAME only if needed
 5. Make sure the runner can run docker build.
 6. Run build-ci-image workflow.
 7. Confirm image appears in Gitea Packages.