Don't escape Docker env file values

lib/kamal/env_file.rb

@@ -23,22 +23,13 @@ def to_io
 
   private
     def docker_env_file_line(key, value)
-      "#{key}=#{escape_docker_env_file_value(value)}\n"
-    end
+      value = value.to_s
 
-    # Escape a value to make it safe to dump in a docker file.
-    def escape_docker_env_file_value(value)
-      # keep non-ascii(UTF-8) characters as it is
-      value.to_s.scan(/[\x00-\x7F]+|[^\x00-\x7F]+/).map do |part|
-        part.ascii_only? ? escape_docker_env_file_ascii_value(part) : part
-      end.join
-    end
+      # Docker env files don't support escape sequences, so newlines and null
+      # bytes cannot be represented. Raise an error rather than silently corrupt.
+      raise ArgumentError, "Env file values cannot contain newlines" if value.include?("\n")
+      raise ArgumentError, "Env file values cannot contain null bytes" if value.include?("\0")
 
-    def escape_docker_env_file_ascii_value(value)
-      # Doublequotes are treated literally in docker env files
-      # so remove leading and trailing ones and unescape any others
-      value.to_s.dump[1..-2]
-        .gsub(/\\"/, "\"")
-        .gsub(/\\#/, "#")
+      "#{key}=#{value}\n"
     end
 end

test/env_file_test.rb

@@ -57,21 +57,21 @@ class EnvFileTest < ActiveSupport::TestCase
       "foo" => "hello\\nthere"
     }
 
-    assert_equal "foo=hello\\\\nthere\n", \
+    # Literal backslash-n should be preserved, not double-escaped
+    assert_equal "foo=hello\\nthere\n", \
       Kamal::EnvFile.new(env).to_s
   ensure
     ENV.delete "PASSWORD"
   end
 
-  test "to_s newline" do
+  test "to_s newline raises" do
     env = {
       "foo" => "hello\nthere"
     }
 
-    assert_equal "foo=hello\\nthere\n", \
+    assert_raises ArgumentError do
       Kamal::EnvFile.new(env).to_s
-  ensure
-    ENV.delete "PASSWORD"
+    end
   end
 
   test "stringIO conversion" do