create_HighSierra_iso.sh

hdiutil create -o /tmp/HighSierra.cdr -size 7000m -layout SPUD -fs HFS+J &&\
hdiutil attach /tmp/HighSierra.cdr.dmg -noverify -mountpoint /Volumes/install_build &&\
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/install_build &&\
mv /tmp/HighSierra.cdr.dmg ~/Desktop/InstallSystem.dmg &&\
hdiutil detach /Volumes/Install\ macOS\ High\ Sierra &&\
hdiutil convert ~/Desktop/InstallSystem.dmg -format UDTO -o ~/Desktop/HighSierra.iso &&\
mv ~/Desktop/HighSierra.iso.cdr ~/Desktop/HighSierra.iso &&\
rm ~/Desktop/InstallSystem.dmg

Download HighSierra dmg file:

Instructions:
http://osxdaily.com/2017/09/27/download-complete-macos-high-sierra-installer/

Short version:

1. Download this: https://drive.google.com/file/d/1Wsbkxqx24SQzRjX_Ng-_H2gQdPW_U70U/view
2. Go to Tools > Download MacOS High Sierra
3. Cancel the rest of the process once the download has completed