Prompt to create dockerfile when one doesn't exist for docker build

If you run `func kubernetes deploy` in a directory you haven't yet generated a docker file, you get this error:

>Error running docker build -t registry/python-function C:\File\Path.
output:

> unable to prepare context: unable to evaluate symlinks in Dockerfile path: CreateFile C:\File\Path\Dockerfile: The system cannot find the file specified.

The fix is simple enough, just run `func init --docker-only` and you have a dockerfile. However would be great if this error either:

1. Suggested running `func init --docker-only` first to generate a Dockerfile
2. Actually prompted if you wanted to generate a Dockerfile first, and then ran that command for you

Start with the `func kubernetes deploy` path that invokes `docker build` when no Dockerfile exists, and compare its failure with `func init --docker-only`. Confirm whether the intended behavior is to suggest the command or prompt and run it; done means the missing-Dockerfile case gives the agreed guidance or prompt instead of the current error.

Written by the indexing model from the issue text.