Upload image from local fileystem

**What is the problem this feature would solve? Please describe.**
Currently the `doctl compute image create` command just allows to create an image based on a URL.
What if I build an image locally or on another droplet. How can I add it as an image?

**Describe the solution you'd like**
Add `--image-path` as an alternative for `--image-url` to upload local files as images.