Chore: improve .env.example for customizable env vars for docker compose

.env.example

@@ -1,14 +1,20 @@
 # your domain, e.g https://example.com
-APP_URL=http://localhost:3000
 PORT=3000
+APP_URL=http://localhost:${PORT}
 
-# make sure to replace this.
+# make sure to replace this : `openssl rand -hex 32`
 APP_SECRET=REPLACE_WITH_LONG_SECRET
 
 JWT_TOKEN_EXPIRES_IN=30d
 
-DATABASE_URL="postgresql://postgres:password@localhost:5432/docmost?schema=public"
-REDIS_URL=redis://127.0.0.1:6379
+POSTGRES_PORT=5432
+POSTGRES_DB=docmost
+POSTGRES_USER=docmost
+POSTGRES_PASSWORD=docmost
+DATABASE_URL="postgresql://${POSTGRES_USER}:${POSTGRES_PASSWORD}@db:${POSTGRES_PORT}/${POSTGRES_DB}?schema=public"
+
+REDIS_PORT=6379
+REDIS_URL=redis://127.0.0.1:${REDIS_PORT}
 
 # options: local | s3
 STORAGE_DRIVER=local