Use SSH RSA key
In !66 (merged) I added code to generate an ed25519 key in addition the RSA key we already generate (if it doesn't exist) for use as the SSH server host key. This is bad since it means the host key will change when going from a version older than 5.3 to 5.3 or newer. The better solution is to simply reconfigure 5.3 to remain using RSA.
Once most supported NSO versions can support ed25519 we could consider changing the default. Before that, it is better to get consistent behavior across the board by doing RSA.
Note how this problem is unique to NSO in Docker. A normal NSO setups writes ncs.conf once upon installation and then uses the same file. Since the key algo is picked on first installation it will be RSA if the installation was done with NSO older then 5.3 and ed25519 if done with NSO 5.3 or later. For NSO in Docker we always generate a configuration upon startup of the container and thus, we will have to set the option to whatever we want it to be. To provide for a consistent user experience, I think we should opt for RSA.