Pass SWISH_NUMBER to production backend container.

Payment instructions need the merchant Swish number from environment
variables, consistent with dev compose.

docker-compose.prod.yml

@@ -29,6 +29,7 @@ services:
       STRIPE_SECRET_KEY: ${STRIPE_SECRET_KEY}
       STRIPE_WEBHOOK_SECRET: ${STRIPE_WEBHOOK_SECRET}
       STRIPE_PRICE_ID: ${STRIPE_PRICE_ID}
+      SWISH_NUMBER: ${SWISH_NUMBER}
     depends_on:
       postgres:
         condition: service_healthy