Hi Matteo,
I’ve not used the realtek wifi dongle in a production environment. I’ve tried to get it working, and you can get it to work using network_mode: host and disable NetworkManager for that device, as described above. I’ve to say, it’s a long time ago, so I don’t remember everything about it.
But in short, from what I can remember, is using the network_mode: host in your container, disable NetworkManager for that device as described above and installing the hostapd fork for the RTL8188eu driver (like lwfinger/rtl8188eu: Repository for stand-alone RTL8188EU driver. (github.com)).
The reason why I didn’t go further with this is because I didn’t think it would work stable enough. I learned a lot about creating an access point and make it manageable (you can read everything here), so maybe with that experience, you can create a much more stable and reliable access point using the Wi-Fi dongle.
I’ve chosen to use the onboard Wi-Fi chip for the Access Point as well as a Wi-Fi client. I’ve created something like the Wi-Fi Connect, but with Node.js and implemented that in my whole application. So when it has Wi-Fi, it’s just a client, and when it can’t connect to Wi-Fi, it’s an AP with captive portal. This removes the need of an extra USB, it just requires a lot of testing and debugging, and it should be suitable for your project of course!
If you need any help, you can always post it here, because you’re not the only one asking about it 
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I’m happy to help debugging any problems you run into by looking at your code. Can’t promise you that I’ll have the solution or if I can try it on my own device, but let me know!