It sounds like @mathieubossaert doesn't need to do form submission for his testing so this isn't as important but I still wanted to follow up because I learned some things discussing with @issa and @Matthew_White.
Using ngrok does work when using the development environment (no Docker). One additional thing to note is that the config/default.json file in the backend folder will need to be modified. In env, domain will need to be set to the public ngrok address.
However, using ngrok is not possible with the Docker setup and that's why @issa detailed the three options above. The most practical is probably to get a throw-away domain name even for local testing (e.g. from https://freedns.afraid.org/) so that you can get an SSL certificate.