feat: Add session secret env var check

[Planlos5000]

Add documentation for the SESSION_SECRET env var.
Also add a check if the variable is set if an authentication provider is configured.
Add SESSION_SECRET to the docker compose.

Description

Add the documentaion for the SESSION_SECRET environment variable.
Also add a check if the SESSION_SECRET is set if an authentication provider is set.

Changelog

Add documentation for the SESSION_SECRET env var.
Add check if the variable is set if an authentication provider is configured.
Add SESSION_SECRET to the docker compose.

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• The light- and dark-theme are both supported and tested
• The design was implemented and is responsive for all devices and screen sizes
• The application was tested in the most commonly used browsers (e.g. Chrome, Firefox, Safari)

[Schwehn42]

lgtm 🙏

[github-actions]

The deployment to the dev cluster was successful. You can find the deployment here: https://4709.development.scrumlr.fra.ics.inovex.io
This deployment is only for testing purposes and will be deleted after 1 week.
To redeploy rerun the workflow.
DO NOT STORE IMPORTANT DATA ON THIS DEPLOYMENT

Deployed Images

• ghcr.io/inovex/scrumlr.io/scrumlr-frontend:sha-db3e11d

• ghcr.io/inovex/scrumlr.io/scrumlr-server:sha-db3e11d

[octomind-dev]

🐙 Octomind

Test Report: 14/14 successful.

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test all ways to open the setup flow	Passed ✅	click

commit sha: db3e11d