ssh key for dgit-repos in t2u container

The dgit inside the container needs to ssh to dgit-repos. dgit-repos ssh login is not a primary access control, so we don't need this to use a hardware token key or anything. Simply copying the key into the container seems fine. I'm relaxed about the possibility of it being exfiltrated by compromised a source package build - even that wouldn't be a disaster, and it doesn't seem very likely anyway.

I think this means we need to provide the container user with a real $HOME.

The alternative would be to somehow offer ssh agent forwarding to the container. I found this: https://stackoverflow.com/questions/75781371/forwarding-ssh-agent-to-container-with-podman-in-fedora It doesn't look easy.