Add grafana to docker-compose

[oscgonfer]

Previously we have been running grafana-agent via systemd, but it would be good if we can add it to the compose.yml file.

As it runs on the mqtt server:

docker run -d --name grafana \
    --restart="unless-stopped" \
    --memory="1g" \
    --memory-swap="1g" \
    -v /tmp/agent:/etc/agent/data \
    -v /root/grafana-agent-config.yaml:/etc/agent/agent.yaml \
    grafana/agent:v0.11.0

However, would be good to make it all run with docker, moving user/passwords over to the .env file (and include them in env.example):
https://grafana.com/docs/grafana/latest/setup-grafana/installation/docker/

So:

• Add grafana to docker-compose.yml with no dependency. Evaluating if most of the agent-config can be moved over to the docker-compose file
• Add credentials to .env and fake ones to env.example

[oscgonfer]

Done by #299