CHARON_BEACON_NODE_HEADERS doesn't pass values

# 🐞 Bug Report

### Description

The CHARON_BEACON_NODE_HEADERS parameter in the sample .env files don't work since the docker-compose.yml doesn't actually grab these values when starting the charon container.

## 🔬 Minimal Reproduction

Setting CHARON_BEACON_NODE_HEADERS (e.g for beacon node auth) won't pass the headers.