Support a local build command

[tazjin]

Nixery should have a local nixery build foo/bar:baz command that spits out an image tarball.

This has been brought up on an external Slack, where someone said that they'd be unlikely to use Nixery unless there was an easy option of running builds locally.

Due to the amount of config Nixery requires at the moment there's a question of whether to recommend direnv or add support for some local configuration file instead.