docs: Adds warning for env injection in docker-compose

[HPaulson]

This PR updates the documentation to make it clear that environment variables set in the Dokploy UI are not automatically injected into containers, unlike commercial alternatives. The has been widely confusing:

• Environment variables from UI not injected into containers after update from 0.22.7 dokploy#2088
• Environment variables not being set in container dokploy#385

This docs change serves as a temporary solution to the above confusion, allowing users can find an answer on the official docs rather than hunting down closed GitHub issues/discussions. However, a complete fix should include:

1. A warning in UI on the variables tab; Or, preferably,
2. Automatic injection of the variables by modifying the docker-compose file (this is already done for Traefik labels, so I'm unsure why it couldn't be done here)

Both of which have been suggested in the issues listed above.

Screenshot of docs fix: