I don't know why you are talking about Docker but even for Docker, I have their public GPG key setup in my puppet installation code. Docker is installed with AP

I don't know why you are talking about Docker but even for Docker, I have their public GPG key setup in my puppet installation code. Docker is installed with APT and using the same signature mechanism. Docker in turn now does container signature verification. There is a chain there. If an attacked wants to compromise that chain he has to be present from the start and re-sign the packages trough MITM all the time. If I switch Internet connections and fetch an update I will get a signature mismatch. Now compare to protonmail's webmail. At any point, if an attacker is able to MITM SSL then the user is compromised. Game over. The client won't even have the chance to see a signature mismatch and take appropriate actions after the fact. I'm not saying that protonmail is a bad idea but "web crypto" definitely is in my book. It doesn't mean you can't implement another client for the desktop like you did for Android and iOS. Distributing the software and the data on different channels really makes the attacks more difficult.