Create Dockerfile with current Ruby version.

[jelaniwoods]

Part of #114

Adds a Dockerfile that has the current version of Ruby the project was generated with.

While entirely possible, I don't think the template should build the docker image and push to Docker hub. It could take too much time also we might add more project specific gems. If that is something we want to do, I think there should be a command or task for it (maybe in the appdev_support gem).

I'm also unsure of whether it makes sense to default the gitpod.yml to use this Dockerfile or not. There is a change of syntax from

image: jelaniwoods/appdev2020-base

to

file: Dockerfile

Maybe that's something for the task that deploys?