Fix for Dotenv 3.1.5

In Dotenv 3.1.5, `Dotenv.parse` no longer returns values that are
already in the environment.

See https://github.com/bkeepers/dotenv/issues/518

We can get the values though by setting overwrite: true, which works
with both 3.1.4 and 3.1.5.

lib/kamal/secrets.rb

@@ -32,7 +32,7 @@ class Kamal::Secrets
   private
     def secrets
       @secrets ||= secrets_files.inject({}) do |secrets, secrets_file|
-        secrets.merge!(::Dotenv.parse(secrets_file))
+        secrets.merge!(::Dotenv.parse(secrets_file, overwrite: true))
       end
     end