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.

config/storage.yml

@@ -6,7 +6,7 @@ test:
   service: Disk
   root: <%= Rails.root.join("tmp/storage") %>
 
-<% if ENV["S3_ENABLED"] %>
+<% if ENV["S3_ENABLED"] && ENV["S3_ENABLED"].to_s != "false" %>
 s3:
   service: S3
   endpoint: <%= ENV["S3_ENDPOINT"] %>