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.

app/models/setting.rb

@@ -15,6 +15,9 @@ class Setting < RailsSettings::Base
   field :redis_url, type: :string,
     default: ENV["REDIS_URL"] || "redis://localhost:6379/0"
 
+  field :s3_enabled, type: :boolean,
+    default: ENV["S3_ENABLED"] && ENV["S3_ENABLED"].to_s != "false"
+
   #
   # Registrations
   #