blobinfocache: correctly detect the UID in a namespace
blobinfocache: correctly detect the UID in a namespace

for tools in containers/ we are using the env variable
_CONTAINERS_ROOTLESS_UID when running in a user namespace to refer to
the rootless UID that created the user namespace.

Closes: https://github.com/containers/libpod/issues/2510#issuecomment-480066763

Signed-off-by: Giuseppe Scrivano <20002b7b24c75512093a71a58285826d5c504fca@redhat.com>

containers/image,containers/image