`mix phx.gen.release --docker` can fail because hexpm/bob skips images

[aymanosman]

hexpm/bob can sometimes skip building images in a way that results in only one architecture being available, so an image might build fine on my laptop and fail to build on my infrastructure. See this.

I see you can explicitly pass the OTP and Elixir version you want after this, so maybe that is the way to go.

Although it is easy to workaround this, maybe worth thinking about a fix for a smoother experience for beginners.