This gist details how to generate a bootable High Sierra installer ISO image. It solves a few differences from the Sierra install. It assumes the Install downloaded into the directory with the script.

create-highsierra-iso.sh

src=./Install\ macOS\ High\ Sierra.app
hdiutil attach \
    ./Install\ macOS\ High\ Sierra.app/Contents/SharedSupport/InstallESD.dmg \
    -noverify -nobrowse -mountpoint /Volumes/install_app

hdiutil create -o /tmp/HighSierra.cdr -size 7316m -layout SPUD -fs HFS+J

hdiutil attach /tmp/HighSierra.cdr.dmg -noverify -nobrowse -mountpoint \
    /Volumes/install_build

# Do something and rename the install_build mount point to "OS X Base System"
asr restore --source "${src}"/Contents/SharedSupport/BaseSystem.dmg \
    --target /Volumes/install_build --noprompt --noverify --erase

# High Sierra uses this link to mount the Packages dir in a ramdisk
cp -rp /Volumes/install_app/Packages /Volumes/OS\ X\ Base\ System/System/Installation/PackagesLink

cp -rp "${src}"/Contents/SharedSupport/BaseSystem.chunklist /Volumes/OS\ X\ Base\ System/BaseSystem.chunklist

cp -rp "${src}"/Contents/SharedSupport/BaseSystem.dmg /Volumes/OS\ X\ Base\ System/BaseSystem.dmg

hdiutil detach /Volumes/install_app

hdiutil detach /Volumes/OS\ X\ Base\ System/

hdiutil convert /tmp/HighSierra.cdr.dmg -format UDTO -o /tmp/HighSierra.iso

mv /tmp/HighSierra.iso.cdr ./HighSierra.iso

rm -f /tmp/HighSierra.cdr.dmg