use an unused config var name

because config vars are currently tenant-scoped, DATABASE_URL from
another sample could override the interpolated value in our compose
file.

Author: jordanstephens

samples/fastapi-postgres/compose.yaml

@@ -9,7 +9,7 @@ services:
         target: 8000
         published: 8000
     environment:
-      - DATABASE_URL=postgres://postgres:${POSTGRES_PASSWORD}@db:5432/postgres
+      - DB_URL=postgres://postgres:${POSTGRES_PASSWORD}@db:5432/postgres
     depends_on:
       - db
     #deploy:

samples/fastapi-postgres/fastapi/main.py

@@ -8,9 +8,9 @@
 from databases import Database
 from dotenv import load_dotenv
 
-load_dotenv() 
+load_dotenv()
 
-database_url = os.getenv("DATABASE_URL")
+database_url = os.getenv("DB_URL")
 print(f"Connecting to database at: {database_url}")
 database = Database(database_url)