docker: Add optional keycloak service and configure its use in swh-web

Add optional keycloak service in order to test user authentication and authorization
with OpenID Connect in other swh services.

Enable OIDC user authentication in swh-web.

To start keycloak related services, use:

$ docker-compose -f docker-compose.yml -f docker-compose.keycloak.yml up -d

Related to T2219