Minor typo fixes and rewording for Dockerfile

Just some small tweaks. Also, note that the [Internet](http://manpages.ubuntu.com/manpages/trusty/en/man8/sshd.8.html) claims that "UseDNS no" may not be enough to stop `sshd` from doing DNS lookups, it might also be necessary to add `-u0` to daemon options to make `sshd` stop issuing DNS requests. I haven't tested this thoroughly, though.