We usually copy an existing one and just edit it to do what we want. I would say a day or two for someone that has never used docker to make a somewhat complex dockerfile

To get something simple running where you start with an existing file and just add your executable to the container, less than an hour.

You should hire somebody to manage the Kubernetes container infrastructure you'll probably end up with. Devops guys handle it where I've worked. Docker on a local machine is relatively simple, but deploying the containers to a production environment using Kubernetes is currently extremely complex