bug: fix container :type => <type>

Copypasta error in #6068 means that `container :type => <type>`
never worked, because it looked for nonexistent method `container.formula`
in `lib/cask/dsl.rb`

This happened because I recklessly pushed through the new `container`
stanza to make 0.40.0 on 8 Sep for inclusion in DSL 1.0.

This PR adds a test.