add our own sshd_config
concatenate our intermediate apt-get steps

...two fewer layers, etc.
add a run/setup wrapper script

...that handles key generation, etc, and the like, for first time runs.  These
are things that we cannot handle in the Dockerfile, as we're going to want
them to be different for each container (e.g. ssh hostkeys and the like).
add a LICENSE

...as I should have done so long ago.
Dockerfile
sshd_config updates
add keys to admin{,.pub}
add default admin.pub