Validate frontend url

# WARNING: Commenting after a variable's value DOES NOT WORK in .env files.
# In other words, don't to this: `FOO=value # this is a comment`.
# This would give the value "value # this is a comment" to the FOO variable.
# You need to use single-line comments instead, e.g:
# ```
#   # this is a comment:
#   FOO=value
# ```

#
#
#

# Frontend app URL and bare domain name:
FRONTEND_URL=https://api.paywithpi.com
FRONTEND_DOMAIN_NAME=paywithpi.com

# Backend app URL and bare domain name:
BACKEND_URL=https://backend.paywithpi.com
BACKEND_DOMAIN_NAME=backend.paywithpi.com


# Obtain the following 2 values on the Pi Developer Portal (open develop.pi in the Pi Browser).

# Domain validation key:Https://calcu-pi.comf7d8531b796b28a6223d7045dc9e437cc6f1a8b07932db982885dc293f6855d55c8cd09739ab0f2e24b74797ee5dbb5a999eba3e602b0d69304f1c3df0eb6ed0
DOMAIN_VALIDATION_KEY=Https://paywithpi.com/validation-key.txt
# Pi Platform API Key:qkyxgarvxnndxhlwg1vw1x7ql2q0bivykfc8zid4ao7jskbv4imimvnn2rft0clr
PI_API_KEY=Https://paywithpi.com/validation-key.txt

# Generate a random string, or roll your face on the keyboard to fill this value:
SESSION_SECRET=SAAYNZRFJIHO33RRMRPRQMBJIGBLX2ROGOH2AIUYNMTQDP6OE3QXJ4RN

# MongoDB database connection details:
MONGODB_DATABASE_NAME=PayWithPiapp
MONGODB_USERNAME=PayWithPiapp
MONGODB_PASSWORD=#Miracle2602


# This will be prepended to all container names.
# Changing this will make docker-compose lose track of all your containers.
# Run `docker-compose down` before changing it.
COMPOSE_PROJECT_NAME=PaywithPi-app

# Set this to either "development" or "production" (XXX "staging"?):
ENVIRONMENT=production

# This directory will be used to store all persistent data needed by Docker (using volumes):
DATA_DIRECTORY=./docker-data

# URL of the Pi Platform API - you should not need to change this.
PLATFORM_API_URL=https://api.mine.com