The OpenSSH version on Ubuntu 12.04 (Precise) is 5.9p1. OpenSSH supports ED25519 format after version 6.5.
To SSH to our 12.04 containers you’ll need an RSA, ECDSA or DSA key.
The 14.04 build image has a newer version of SSH so all key types are supported there.
(Thanks to this post by @Saneyan for originally identifying this)