Enhancements - Fetch env vars dynamically.

Great tool, thank you for this. Although during local deployment I noticed some issues.

• Despite providing Github PAT, the code fetches the PAT not from .env but from inside the "Private Repos" dialog box where we are prompted to enter the key. Its visible in src/hooks/useDiagram.ts where the github token if not given is set to undefined
• Similarly, a suggestion would be to define BACKEND_PORT var in .env for easy fetching by /backend/entrypoint.sh and /backend/Dockerfile. I had to change the port values in both files.