ENV Substitution

[ProbablePrime]

The following two items here don't support ENV var substitution:

1. Prometheus' scrape config: https://github.com/Yellow-Dog-Man/monitoring-compose/blob/beep/prometheus/prometheus.yml
2. Dmarc's inbox config: https://github.com/Yellow-Dog-Man/monitoring-compose/blob/beep/dmarc-reporting/resonite/dmarc-metrics-exporter.json

Therefore I keep getting git conflicts when working with this because the production source has those filled out.

Prometheus is very adamant in it's stance of not adding env substitution to its stack. so https://linuxhandbook.com/envsubst-command/ might be a good approach. for that.

As for DMarc, I'll probably see if I can PR the support.

[ProbablePrime]

prometheus added a "password file" function that allows me to replace passwords with files that contain the passwords. This solved the prometheus one.

As for Dmarc still need to get that sorted.