I am making a video game in Windows, and I would like to package it for Windows 10 and Apple MacOS (so in the .exe and .app format, or anything similar that is compatible with the two platforms) so that everyone can play.
I cannot seem to find any way to create a .app file or other MacOS-compatible format on Windows, and I can't seem to make a .exe into a .app.
Is it possible to turn Docker image into a .app file?
Docker isn't an appropriate packaging format for this sort of application.
A Docker image requires the Docker runtime to be runnable; it will not be standalone in the way you're expecting. If you're making a video game, you also need to access the host hardware; if you're running a Linux container then you also need an X server on the host system, which can't be provided in Docker. The setup to run this will be complex and requires administrator-level permission to run.
I'd recommend building a truly native application, and using native packaging tools to distribute it. This will involve having a development system for each platform you want to target. You'll need this anyways for testing, and it will be significantly less complex than inserting an isolation system like Docker in the middle of your application.