Fix Build and push complement image CI job pointing to non-existent image (#19523)

❌
https://github.com/element-hq/synapse/actions/runs/22609655282/job/65509315002#step:8:39
```
Error response from daemon: No such image: complement-synapse:latest
```

Regressed in
https://github.com/element-hq/synapse/pull/19475#discussion_r2823157623
where we updated `complement.sh` to build `localhost/complement-synapse`
instead of `complement-synapse`.

.github/workflows/push_complement_image.yml

@@ -69,6 +69,7 @@ jobs:
         run: |
           for TAG in ${{ join(fromJson(steps.meta.outputs.json).tags, ' ') }}; do 
             echo "tag and push $TAG"
+            # `localhost/complement-synapse` should match the image created by `scripts-dev/complement.sh`
             docker tag complement-synapse $TAG
             docker push $TAG
           done