Environment Variables
OS environment variables used for consistency between updates through vcs and security.
How to set environment variables permanently in Ubuntu-20.04?
Do not edit /etc/environment or /etc/profile or /etc/bash.bashrc.
Create a file named freightapp.sh in /etc/profile.d. 
Put appropriate values in repo/freightapp-envvars.sh and copy it to /etc/profile.d.
# assuming you are in the repo root directory
sudo cp freightapp-envvars.sh /etc/profile.d/freightapp.sh
There put the values as:
# inside /etc/profile.d/freightapp.sh
export ENVVARS=value
- Beware of sudo caveat.
 - Server needs logout-login to take in effect
 
Details at Ubuntu doc page
FREIGHTAPP_SECRET_KEY
it is the value for settings.SECRET_KEY. Which currently looks like: SECRET_KEY = os.environ.get('FREIGHTAPP_SECRET_KEY')
Generate your secret key with ROOT/tools/secret_key.py
# execute in the project root
python tools/secret_key.py
# output: set settings.SECRET_KEY=<secret_key>
Now set the environment variable with the generated secret key.
#inside /etc/profile.d/freightapp.sh
export FREIGHTAPP_SECRET_KEY=<secret_key>
#remove the curly braces
FREIGHTAPP_SENTRY_DSN
Check sentry integration page to get your dsn. Then follow as above.
FREIGHTAPP_DATABASE_URL
Database url(dsn) for DOKKU deployment. Check related page.