Fix prometheus in local metrics docker setup

[nflaig]

Motivation

Dockerized setup to collect metrics from local beacon node and validator client is broken

Description

need to set in environment now instead of args else default values set in Dockerfile will be used

lodestar/docker/prometheus/Dockerfile

Lines 11 to 13 in d5e05d3

	ENV BEACON_URL='beacon_node:8008'
	ENV VC_URL='validator'
	VOLUME /prometheus

also fixed default value for VC_URL to use default metric port

One other thing, the local grafana docker build is also broken but not sure how to go about that and I think the issue is already known

this is not docker build compliant

lodestar/docker/grafana/Dockerfile

Lines 10 to 11 in d5e05d3

	# copy over some reasonable default dashboards, user can mount a volume over top of it
	COPY --from=dashboards . /dashboards/

should not rely on buildx features here if possible

[wemeetagain]

Agree the reliance on buildx is not good. Open to suggestions to make life easier

[nflaig]

@wemeetagain I am not sure if there is a approach without some trade-offs, at the moment running docker-compose -f docker/docker-compose.local.yml up -d will fail due to as it tries to load dashboards from docker.io/library/dashboards:latest.
To make the build work have to comment out this line COPY --from=dashboards . /dashboards/ which is not ideal.

I see two possible solutions for this

1. move the build context to root folder which would allow to copy dashboards without relying on buildx
2. publish a separate image chainsafe/lodestar-dashboards which just bundles the dashboards then load the dashboards from that image. This image could potentially also be useful for 3rd party services which want to make use of lodestar dashboards in their custom builds.

[wemeetagain]

🎉 This PR is included in v1.6.0 🎉