You are not required to release the build tools for the image. I would argue that the image contains/aggregates various other software and just arranges them in a way that allows the image to be used for booting. An .iso file is not fundamentally distinct from a ZIP or TAR archive. You therefore have to comply with the licenses for all included software. For binaries that are under various GPL versions (e.g. the GNU userland and the Linux kernel) this requires you to offer the corresponding source.
But your image is not derivative of that included software. The image can also include proprietary software and be itself distributed under a proprietary license, as long as that doesn't violate the license for the GPL-licensed components. A classic example of this is any proprietary Android ROM.
Note that the GPL never forces you to publish any software that you wrote. However, if something you write is derivative of a GPL'ed work, then you can only publish it under the terms of the GPL. If you publish binaries of GPL software you have to offer the Corresponding Source which includes build scripts etc. so that users can exercise their right to make their own changes to the software. But here those GPL'ed binaries are inside the image, and not the image itself.