Improve documentation around secret in docker compose file

[gilgougnot]

In the documentation of the docker compose file, they mention the usage of a secret that we need to generate.
Secrets can not be in clear text in the docker compose file.

The documentation does not mention what is the usage of that secret, how the application work with it, and does not mention a secure way to handle it.

Requesting to improve the installation guidelines to add a quick note about that secret, usage, requirement, constraint and how to secure it.