ostree can't resolve hostnames due to empty resolv.conf

[dsd]

flatpak can't do much:

$ flatpak -vv remote-info flathub org.chromium.Chromium
F: Fetching summary index file for remote ‘flathub’
F: Loading https://dl.flathub.org/repo/summary.idx using curl
F: Should retry request (remaining: 5 retries), due to transient error: While fetching https://dl.flathub.org/repo/summary.idx: [6] Could not resolve hostname
F: Loading https://dl.flathub.org/repo/summary.idx using curl
F: Should retry request (remaining: 4 retries), due to transient error: While fetching https://dl.flathub.org/repo/summary.idx: [6] Could not resolve hostname
F: Loading https://dl.flathub.org/repo/summary.idx using curl
F: Should retry request (remaining: 3 retries), due to transient error: While fetching https://dl.flathub.org/repo/summary.idx: [6] Could not resolve hostname
F: Loading https://dl.flathub.org/repo/summary.idx using curl
F: Should retry request (remaining: 2 retries), due to transient error: While fetching https://dl.flathub.org/repo/summary.idx: [6] Could not resolve hostname
F: Loading https://dl.flathub.org/repo/summary.idx using curl
F: Should retry request (remaining: 1 retries), due to transient error: While fetching https://dl.flathub.org/repo/summary.idx: [6] Could not resolve hostname
F: Loading https://dl.flathub.org/repo/summary.idx using curl
F: Failed to download optional summary index: While fetching https://dl.flathub.org/repo/summary.idx: [6] Could not resolve hostname
error: Unable to load summary from remote flathub: While fetching https://dl.flathub.org/repo/summary.idx: [6] Could not resolve hostname

I think the underlying cause is ostree:

# ostree pull eos
error: While fetching https://ostree.endlessm.com/staging/dev/eos/summary.sig: [6] Could not resolve hostname

Possibly the same as: flatpak/flatpak#5943

Spotted that /etc/resolv.conf was empty:

$ cat /etc/resolv.conf 
# Generated by NetworkManager

Added a nameserver:

$ cat /etc/resolv.conf 
# Generated by NetworkManager
nameserver 8.8.8.8

and now the problem goes away.

cc @dbnicholson