Push <image>:latest in addition to <image>:<git-ref>

[tbuehlmann]

1ed4a37 introduced pulling <image>:latest in addition to <image>:<git-ref>:

https://github.com/mrsked/mrsk/blob/cec82ac641056dce1718f4cbdc99edcd8bc97484/lib/mrsk/commands/builder/base.rb#L8-L11

As we're not pushing <image>:latest:

https://github.com/mrsked/mrsk/blob/cec82ac641056dce1718f4cbdc99edcd8bc97484/lib/mrsk/commands/builder/native.rb#L10-L14

… we're getting an error when trying to deploy:

ERROR (SSHKit::Command::Failed): docker exit status: 1
docker stdout: Nothing written
docker stderr: Error response from daemon: manifest for <registry>/<project>/<app>:latest not found: manifest unknown: manifest unknown

So this PR adds pushing <image>:latest.

[dhh]

cc @djmb