In the beginning of the year 2019 I already wrote that Pharo is able to bootstrap the image. This is based on the Pharo Bootstrap project from Guille and already used in the Pharo Bootstrap process itself to form images like Pharo 7 and Pharo 8 for the releases.
If you look in the Pharo project repository you will find a bootstrap folder with the code that is used to bootstrap the standard pharo image if you are interested.
But sometimes you want to build and bootstrap an own fully custom image. There are new tools helping you to do this like the nice Pharo Bootstrapper project. This project allows you to use Pharo build small images and bootstrap own Smalltalk systems if you like.
I can also recommend to read the paper: "Challenges in Debugging Bootstraps of ReflectiveKernels"