Avoid Docker in restore-external-snapshot

Resolves #86 (closed).

This originally used xq, but inconsistent behavior between versions caused confusion for nodes. It was updated to use docker to avoid local versioning issues, but nodes complain they do not want Docker on their deployment box. This updates to use xmllint provided by libxml2, which hopefully has relatively stable behavior across versions and should be a trivial package install (and likely a system default).