Add secrets-common for shared secrets

Add a shared secrets file used across all destinations. Useful for
things Github tokens or registry passwords.

The secrets are added to a new file called `secrets-common` to highlight
they are shared, and to avoid acciedentally inheriting a secret from the
`secrets` file to `secrets.destination`.

lib/kamal/secrets.rb

@@ -1,19 +1,20 @@
 require "dotenv"
 
 class Kamal::Secrets
-  attr_reader :secrets_file
+  attr_reader :secrets_files
 
   Kamal::Secrets::Dotenv::InlineCommandSubstitution.install!
 
   def initialize(destination: nil)
-    @secrets_file = [ *(".kamal/secrets.#{destination}" if destination), ".kamal/secrets" ].find { |f| File.exist?(f) }
+    @secrets_files = \
+      [ ".kamal/secrets-common", ".kamal/secrets#{(".#{destination}" if destination)}" ].select { |f| File.exist?(f) }
   end
 
   def [](key)
     secrets.fetch(key)
   rescue KeyError
-    if secrets_file
-      raise Kamal::ConfigurationError, "Secret '#{key}' not found in #{secrets_file}"
+    if secrets_files
+      raise Kamal::ConfigurationError, "Secret '#{key}' not found in #{secrets_files.join(", ")}"
     else
       raise Kamal::ConfigurationError, "Secret '#{key}' not found, no secret files provided"
     end
@@ -25,14 +26,8 @@ class Kamal::Secrets
 
   private
     def secrets
-      @secrets ||= parse_secrets
-    end
-
-    def parse_secrets
-      if secrets_file
-        ::Dotenv.parse(secrets_file)
-      else
-        {}
+      @secrets ||= secrets_files.inject({}) do |secrets, secrets_file|
+        secrets.merge!(::Dotenv.parse(secrets_file))
       end
     end
 end