Sign dmg images (not just their contents)

Since macOS 10.11.5 there is the option to sign the dmg images themselves (not just their contents) to make sure the .dmg file is actually coming from us. Might be worth doing given that the OpenPGP part requires yet another non-native tool for verification while users could use the built-in macOS capabilities to check whether the .dmg is good.

Apart from that I am not sure about the benefit of signing the .dmg itself. Thanks to juno_hacker at HackerOne for pointing out the missing container signature.