docs: mention ARGILLA_AUTH_SECRET_KEY in docker-compose deployment

[davidefiocco]

Description

I'm proposing to add a mention to the var ARGILLA_AUTH_SECRET_KEY introduced in #4539 to the docker-compose-related documentation in docs/_source/getting_started/installation/deployments/docker_compose.md

As far as I am aware, that env var needs to be set in order to run the service in docker-compose.

It's a small edit, I didn't create (yet) an issue for this.

• Documentation update

How Has This Been Tested

• sphinx-autobuild

I didn't run sphinx! I had troubles w/ cloning, will update this as soon as I can

Checklist

• I added relevant documentation
• I followed the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• My changes generate no new warnings
• I filled out the contributor form (see text above)
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[davidberenstein1957]

@davidefiocco thanks for the PR. I updated the phrasing a bit and will merge it now :)