I've also only ever done it once before. Niche use case and not that well known.
This is a networking question and not something that the app could circumvent. You still need to be able to connect to your docker machine, regardless of how you are accessing it. Same as the Blue Iris app.
For now, you should use a VPN to connect to your home network from the internet. It is 100% a very bad idea to expose the app to the internet. That means in any form (ngrok, reverse proxy, port forward, etc). Don't do any of those.
I have been extremely lax with the security so far. It is on my next-up list of things to improve. Once I get to that, you could expose it to the internet if you really wanted to, although it's still better not to.
If you were to expose it to the internet right now, it's not likely to be a situation where some hacker is going to gain access to your whole network and wreak havoc (certainly still possible though). It would be more like you go to log in and the whole thing is broken or all your plate records are gone. I would give a 50+% chance of all your data getting ruined by some automated vulnerability checker in some foreign country if you were to expose it. It really isn't an "oh it's probably better not to..." / it's more principled and better security practice type situation. It legitimately will get hacked. So, VPN for now; security hardening and expose at your discretion coming soon.