Small issues with docker compose instructions

[daniele-sartiano]

I'm testing Mercator, thank you for your work!
I just want to report some minor improvements for the documentation.
Following the instructions in the README under the "With Docker Compose" section, I encountered two issues:

1. Incorrect Prometheus volume path:

The docker-compose.yml file specifies an incorrect path for the Prometheus volume.
I had to adjust the path as follows:

diff
-      - ./prometheus:/etc/prometheus
+      - ./observability/prometheus:/etc/prometheus

2. Docker secret creation requires Docker Swarm

The provided command to create the Docker secret in the docker-compose.yml:

echo "yourusername" | docker secret create grafana-admin-username -

requires Docker Swarm to be enabled. Since Docker Swarm was not enabled in my environment, I reverted to using a file-based secret as shown in previous versions:

file: ~/.env.grafana-username

It could be helpful to document clearly both alternatives: one using Docker Swarm and another without it.