The jumphost container is correctly able to resolve that hostname. Likewise is the container host. So #DNS works correctly in both cases. (I'm still not convinced it's not DNS, yet. Because it's always DNS.)
When I try to `ssh target.example.org` on the container host, that ssh connection works.
When I try to `ssh target.example.org` in the jumphost container, ssh tells me
ssh: could not resolve hostname target.example.org: Name does not resolve
Likewise sshd when jumping.