Before this change, Ironic did not filter file:// paths when used as an
image source except to ensure they were a file (and not, e.g. a
character device). This is problematic from a security perspective
because you could end up with config files from well-known paths being
written to disk on a node.

The allowlist default list is huge, but it includes all known usages of
file:// URLs across Bifrost, Ironic, Metal3, and OpenShift in both CI
and default configuration.

For the backportable version of this patch for stable branches, we have
omitted the unconditional block of system paths in order to permit
operators using those branches to fully disable the new security
functionality.