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.

lib/postal/config_schema.rb

@@ -68,7 +68,8 @@ module Postal
 
       string :signing_key_path do
         description "Path to the private key used for signing"
-        default "config/postal/signing.key"
+        default "$config-file-root/signing.key"
+        transform { |v| Postal.substitute_config_file_root(v) }
       end
 
       string :smtp_relays do
@@ -253,12 +254,14 @@ module Postal
 
       string :tls_certificate_path do
         description "The path to the SMTP server's TLS certificate"
-        default "config/postal/smtp.cert"
+        default "$config-file-root/smtp.cert"
+        transform { |v| Postal.substitute_config_file_root(v) }
       end
 
       string :tls_private_key_path do
         description "The path to the SMTP server's TLS private key"
-        default "config/postal/smtp.key"
+        default "$config-file-root/smtp.key"
+        transform { |v| Postal.substitute_config_file_root(v) }
       end
 
       string :tls_ciphers do
@@ -502,4 +505,14 @@ module Postal
     end
   end
 
+  class << self
+
+    def substitute_config_file_root(string)
+      return if string.nil?
+
+      string.gsub(/\$config-file-root/i, File.dirname(Postal.config_file_path))
+    end
+
+  end
+
 end