Pulp-container: registry credentials are reused across remotes in a worker

A flaw was found in pulp-container when it authenticates to an upstream registry. Basic and bearer credentials from one remote are reused for later downloads in the same worker. A user who can sync a container remote, and can point that remote at a server they control, receives the username, password, or bearer token stored for a different remote, and can reuse that credential at the upstream registry. Content stored in Pulp is not changed, and the service is not stopped.