Substitute env variables in prometheus.yml

[KaloyanTanev]

🎯 Problem to be solved

Prometheus does not substitute environment variables by default. There is an option to do so only for variables external_labels. We have a $PROM_REMOTE_WRITE_TOKEN env var in the prometheus.yml file which is misleading, as it doesn't get expanded when prometheus is started.

🛠️ Proposed solution

We can run a custom script on start of the docker-compose, similarly to what we do with lodestar. There we can expand the variables and then start prometheus itself.