It's actually a relatively difficult thing to bolt on to some apps, their custom domain setup is not very easy. (shameless plug) We let you put custom hostnam

It's actually a relatively difficult thing to bolt on to some apps, their custom domain setup is not very easy. (shameless plug) We let you put custom hostnames + ssl in front of hosted apps, we have a really early attempt at a medium one in progress. If you'd like to try it, send me an email at kurt@fly.io (/shameless plug)