Update Envoy docker compose example

[s-westphal]

• Use official docker images from the GRR and Fleetspeak GitHub container registries.
• Use the GRR docker image and repack and install the client for the client container.
• Updated the used certificates, Client.executable_signing_public_key and PrivateKeys.executable_signing_private_key are now used instead of CA.certificate, PrivateKeys.ca_key, PrivateKeys.server_key and Frontend.certificate.