I am familiar with using the debian netinstaller images for stretch and earlier versions and they boot up with installation of debian as their primary purpose but after the installation is complete, there is a rescue screen in which one can run a shell and a few utilities to mount the boot drive and maybe do fsck and so forth.
Maybe I am missing something obvious but is there a way to boot the CD, select language and keyboard and then skip directly to the rescue shell? I actually downloaded a dedicated rescue disk and successfully used it for a few years but it doesn't understand the ext4 file system used in stretch. The netinstall image puts that FS on the boot drive so it should be able to mount /dev/sda1 if one so desires. When booting the netinstall image, one is prompted to choose locale-specific settings for language and keyboard and then one appears to have no alternative but to format the boot drive and install the OS. I might want to use dd to copy a boot image of another linux system from a usb drive to /dev/sda or I might just want to run fsck -f -y /dev/sda1 if that seems to be necessary. It is always good if you can do things to the boot drive while it is unmounted with no active file system. Thank you. Martin McCormick