Use env vars for registry credentials in kp import

[ncarlson]

kp import will attempt to read registry credentials from these environment variables as a fallback to docker/.config during kp import.

Examples:

	# Passing a single registry credential using environment variables
	REGISTRY_USER=bob REGISTRY_PASSWORD=ross REGISTRY_URL=single-registry.io \
	kp import -f descriptor.yaml

	# Passing multiple registry credentials using the _N suffix
	REGISTRY_USER_0=pablo REGISTRY_PASSWORD_0=picasso REGISTRY_URL_0=first-registry.io \
	REGISTRY_USER_1=henri REGISTRY_PASSWORD_1=matisse REGISTRY_URL_1=another-registry.io \
	kp import -f descriptor.yaml

[tomkennedy513]

We will also need to add docs to the help message so that users know how to utilize this feature

[chenbh]

I agree with #251 (comment) that this should apply to every command that interacts with a registry

[ncarlson]

dockercreds.NewKeychainFromDefaultEnvVarsWithDefault().Keychain is a bit of a mouthful, and any/all suggestions are welcome :-)

[ncarlson]

We will also need to add docs to the help message so that users know how to utilize this feature

Addressing this now...

[tomkennedy513]

This is great!

[chenbh]

Do these need to be exported?

[chenbh]

If I'm reading this correctly, the whole point of this file is to expose the newCredHelperFromEnvVars function for tests? In that case a better idea is to just use package dockercreds in cred_helper_test.go.

The way go works, this would expose the function to any tests, including ones from outside this module.

[chenbh]

FYI, I've found testing to be somewhat inconsistent across even files of the same project.

The rule of thumb is that if you purely want to test public functions, then you can use package my_module_test, if you want to test internals, then it's perfectly fine to use package my_module inside a *_test.go file

[chenbh]

You don't need to worry about cleaning up env vars since child processes cannot interact with the parent's env vars (other than inheriting a copy).

For example, go run-ing this program won't unset $HOME nor add $FOOBAR to the shell

package main
import "os"
func main() {
	os.Setenv("FOOBAR", "BAZ")
	os.Unsetenv("HOME")
}