Update Docker container of yake-server

Hey!

Docker container of `yake-server` is still on 0.4.3.

There's already a release 0.4.4 here.


I see that the Dockerfile uses `master` to build.
There are changes in `master` that haven't been built into a new release yet.
Are those pending changes stable enough? Is it safe to use whatever is currently in `master`, if I want to build a local image with the latest code?

Thank you!