Is that $20/mo per app? I'm assuming if you had your own domain Docker will not be covering the DNS registry and such, effectively making it $30/mo to host a low-traffic app landing page. Honestly, I'm not sure that I would pay that much for my small apps (seeing as these are hobbies).
If it's $20/mo for, say, 5 apps, I'd be much more inclined.
Have you considered anything like a landing page builder that you can host yourself? Just a thought.
Take this all as conjecture. I hope you guys really make a sound product because this has been a serious frustration and pain point for me.
edit: Another feature that would be nice: quotes and links to reviews so you can show off any press or praise that the app has gotten.