Escape newlines in docker env files

When env variables were passed via `-e` newlines were escaped. This
updates the env file to do the same thing.

lib/kamal/utils.rb

@@ -92,6 +92,13 @@ module Kamal::Utils
       .gsub(DOLLAR_SIGN_WITHOUT_SHELL_EXPANSION_REGEX, '\$')
   end
 
+  # Escape a value to make it safe to dump in a docker file.
+  def escape_docker_env_file_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(/\\"/, "\"")
+  end
+
   # Abbreviate a git revhash for concise display
   def abbreviate_version(version)
     if version
@@ -109,10 +116,6 @@ module Kamal::Utils
   end
 
   def docker_env_file_line(key, value)
-    if key.include?("\n") || value.to_s.include?("\n")
-      raise ArgumentError, "docker env file format does not support newlines in keys or values, key: #{key}"
-    end
-
-    "#{key.to_s}=#{value.to_s}\n"
+    "#{key.to_s}=#{escape_docker_env_file_value(value)}\n"
   end
 end