fix: don't override paths in dockerfile

This allows for these paths to continue to be set in the config file or environment variable while still maintaining the default of having the default paths in the same directory as the postal config file.

spec/lib/postal/legacy_config_source_spec.rb

@@ -17,7 +17,9 @@ module Postal
       # by the schema itself. Otherwise, we might see a value returned that
       # looks correct but is actually the default rather than the value from
       # config file.
-      allow_any_instance_of(Konfig::SchemaAttribute).to receive(:default).and_return(nil)
+      allow_any_instance_of(Konfig::SchemaAttribute).to receive(:default) do |a|
+        a.array? ? [] : nil
+      end
     end
 
     let(:source) { described_class.new(SOURCE_CONFIG) }