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
#19475 (comment)
where we updated `complement.sh` to build `localhost/complement-synapse`
instead of `complement-synapse`.

Author: MadLittleMods

.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

changelog.d/19523.bugfix

@@ -0,0 +1 @@
+Fix `Build and push complement image` CI job pointing to non-existent image.

scripts-dev/complement.sh

@@ -38,17 +38,22 @@ set -e
 # Tag local builds with a dummy registry namespace so that later builds may reference
 # them exactly instead of accidentally pulling from a remote registry.
 #
-# This is important as some storage drivers/types prefer remote images over local
-# (`containerd`) which causes problems as we're testing against some remote image that
-# doesn't include all of the changes that we're trying to test (be it locally or in a PR
-# in CI). This is spawning from a real-world problem where the GitHub runners were
+# This is important as some Docker storage drivers/types prefer remote images over local
+# (like `containerd`) which causes problems as we're testing against some remote image
+# that doesn't include all of the changes that we're trying to test (be it locally or in
+# a PR in CI). This is spawning from a real-world problem where the GitHub runners were
 # updated to use Docker Engine 29.0.0+ which uses `containerd` by default for new
 # installations.
+#
+# XXX: If the Docker image name changes, don't forget to update
+# `.github/workflows/push_complement_image.yml` as well
 LOCAL_IMAGE_NAMESPACE=localhost
 
 # The image tags for how these images will be stored in the registry
 SYNAPSE_IMAGE_PATH="$LOCAL_IMAGE_NAMESPACE/synapse"
 SYNAPSE_WORKERS_IMAGE_PATH="$LOCAL_IMAGE_NAMESPACE/synapse-workers"
+# XXX: If the Docker image name changes, don't forget to update
+# `.github/workflows/push_complement_image.yml` as well
 COMPLEMENT_SYNAPSE_IMAGE_PATH="$LOCAL_IMAGE_NAMESPACE/complement-synapse"
 
 SYNAPSE_EDITABLE_IMAGE_PATH="$LOCAL_IMAGE_NAMESPACE/synapse-editable"