[Fly.io] Image does not need to be public

[userbradley]

In the docs the below is stated:

ko/docs/deployment.md

Lines 27 to 33 in f37f2d1

	### [fly.io](https://fly.io)
	```plaintext
	flyctl launch --image=$(ko build ./cmd/app)
	```
	> 💡 **Note:** The image must be publicly available.

This is not correct on the image needing to be public. You are able to use Fly.io's docker registry, and push by logging in with fly auth docker

I guess it's out of scope for this document to include that, but I thought it's something worth raising.

If you have any idea how this could be better explained in the docs, let me know and I'll open a PR

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Keep fresh with the 'lifecycle/frozen' label.

[imjasonh]

Sorry for missing this! I can update the docs to make this clearer.

[userbradley]

Sure,I'm also happy to open a PR for this too.

All the user needs to do is run fly auth docker and then they get the creds for pushing. From there they just set the KO_DOCKER_REPO env variable to be the name of their project.

Users can push to registry.fly.io/<app-name>:sha and then use the deploy command as a second step, or they can do (What you already have) as flyctl launch --image=$(ko build ./cmd/app)

Let me know if you'd like more clarification, or for me to submit a PR