Okay, this makes total sense! Thank you for pointing this out. But it is not via redirection as mentioned in the article, it is because browsers preload HSTS on .dev domains. As discussed in the thread you sent: Why is *.fly.dev redirecting to https automatically?.
I am running multiple test images and open source pre-packaged software from DockerHub. This is why it is really sad to have to make custom images for it or have to stick a reverse proxy in front.