Parametrize container names

There's a corner case in which we would want to run two separate promtail's containers: when pushing logs to more than one Loki instance:

> It is generally recommended to run multiple Promtail clients in parallel
> if you want to send to multiple remote Loki instances.

[Source](https://grafana.com/docs/loki/latest/clients/promtail/configuration/)

To accomplish that, we need to be able to change the default container name in the `docker-compose.yml.j2` template.

Start with the docker-compose.yml.j2 template and inspect how the Promtail container name is currently defined. Trace the role variables used to render this template, then verify that separate rendered configurations can use distinct Promtail container names when targeting multiple Loki instances.

Written by the indexing model from the issue text.