Docker Compose stack to help testing OIDC scenarios in Synapse.

This deploys:

• two Element Web, one on http://localhost:8081/, the other on http://localhost:8082/
• a PG database for Keycloak
• a Keycloak instance on http://localhost:8080. Admin credentials: admin / admin

Keycloak can access the host via host.docker.internal, so whenever Keycloak needs direct access to Synapse (like when sending OIDC Back-Channel Logout tokens) it should use that address.