Rsync in build.snapcraft.io build fails at getaddrinfo

I have a Snap where I’m pulling some media using rsync. When building on build[.]snapcraft[.]io it fails with
rsync: getaddrinfo: amber.worldforge.org 873: Name or service not known

The full build log is at https://build.snapcraft.io/user/worldforge/cyphesis/734936
The server has existed for a while.
Is there some issues with rsync on the build environment?