Be able to test /login/sso/redirect in Complement (#17986)

Be able to test `/login/sso/redirect` in Complement

Spawning from
https://github.com/element-hq/sbg/pull/421#discussion_r1854926218 where
we have a proxy that intercepts responses to
`/_matrix/client/v3/login/sso/redirect(/{idpId})` in order to upgrade
them to use OAuth 2.0 Pushed Authorization Requests (PAR). We have some
Complement tests in that codebase that go over this flow and these
changes are required [in order for the URL's to line
up](https://github.com/element-hq/synapse/blob/d648c8ce3f4cbf61191b9f5302e405f7b0288677/synapse/rest/client/login.py#L652-L673).

scripts-dev/complement.sh

@@ -195,6 +195,10 @@ if [ -z "$skip_docker_build" ]; then
         # Build the unified Complement image (from the worker Synapse image we just built).
         echo_if_github "::group::Build Docker image: complement/Dockerfile"
         $CONTAINER_RUNTIME build -t complement-synapse \
+            `# This is the tag we end up pushing to the registry (see` \
+            `# .github/workflows/push_complement_image.yml) so let's just label it now` \
+            `# so people can reference it by the same name locally.` \
+            -t ghcr.io/element-hq/synapse/complement-synapse \
             -f "docker/complement/Dockerfile" "docker/complement"
         echo_if_github "::endgroup::"