fix: Correct client ping endpoint

[olemathias]

This is to solve #276

Before:
Could not get tags from registry: unauthorized: authentication required

Ping in registry/client.go gets a 404 from https://ghcr.io/v2
While https://registry-1.docker.io/v2 get a 301 redirect to /v2/ (That I assume the client follows)

So just a extra / at the end of the "ping" url and it's working with GitHub Container Registry.

Tested DockerHub and GitHub Container Registry.

[jannfis]

LGTM. thanks @olemathias !