Allow disabling S3 explicitly, disable in Docker Compose

For example when there is a .env.development for running the app on a
host machine directly, but as a developer you also want to run it with
Docker Compose from time to time.

docker-compose.yml

@@ -44,6 +44,7 @@ services:
       REDIS_URL: redis://redis:6379/0
       RS_REDIS_URL: redis://redis:6379/1
       RS_STORAGE_URL: "http://localhost:4567"
+      S3_ENABLED: false
     depends_on:
       - ldap
       - redis
@@ -67,6 +68,7 @@ services:
       REDIS_URL: redis://redis:6379/0
       RS_REDIS_URL: redis://redis:6379/1
       RS_STORAGE_URL: "http://localhost:4567"
+      S3_ENABLED: false
     depends_on:
       - ldap
       - redis