taskgraph build-image fails for images that use $DOCKER_IMAGE_PARENT

[ahal]

The root issue here is that the build-image tasks and taskgraph build-image, build images two separate ways. The former uses the image_builder image (which is defined in Gecko and imo should live in this repo.. but that's another story). This image_builder image uses kaniko and a bit of custom Rust code, and is where the $DOCKER_IMAGE_PARENT variable is processed. The latter simply shells out to docker build. So obviously, that's not going to recognize our custom parent syntax.

Instead, taskgraph build-image should also use the image_builder image and try to replicate what the build-image tasks do as much as possible.