handle unsupported schema correctly

[dungdm93]

I try to create Docker image with this apko.yaml config:

contents:
  repositories:
    - https://packages.wolfi.dev/os
    - http://172.18.0.2:8081/repository/apk
  keyring:
    - https://packages.wolfi.dev/os/wolfi-signing.rsa.pub
    - http://172.18.0.2:8081/repository/apk/melange.rsa.pub
  packages:
    ....

And got weird error message:

2024/03/18 14:32:13 INFO Building images for 1 architectures: [amd64]
Error: initializing apk: failed to initialize apk keyring: failed to read apk key: open http://172.18.0.2:8081/repository/apk/melange.rsa.pub: no such file or directory
2024/03/18 14:32:14 INFO error during command execution: initializing apk: failed to initialize apk keyring: failed to read apk key: open http://172.18.0.2:8081/repository/apk/melange.rsa.pub: no such file or directory

This is because this code tread all non-https URL as file:

go-apk/pkg/apk/implementation.go

Lines 376 to 385 in bd22a93

	if strings.HasPrefix(element, "https://") {
	asURL, err = url.Parse(element)
	} else {
	// Attempt to parse non-https elements into URI's so they are translated into
	// file:// URLs allowing them to parse into a url.URL{}
	asURL, err = url.Parse(string(uri.New(element)))
	}
	if err != nil {
	return fmt.Errorf("failed to parse key as URI: %w", err)
	}

and this line is never reached:

go-apk/pkg/apk/implementation.go

Lines 427 to 428 in bd22a93

	default:
	return fmt.Errorf("scheme %s not supported", asURL.Scheme)

[dungdm93]

cc @jonjohnsonjr, @tuananh