Add GEANT and USERTrust CA certificates to Keycloak container

• Since keycloak communicates with the auth-proxy, trust issues can arise (TUW certs are signed by GEANT)
• To do that, we now switch to a custom image build for keycloak
• The custom image name includes the version number of the actual keycloak version to be pulled (it is an env variable and passed as an argument to the Dockerfile for pull)