I don’t exactly recall when Apple’s Migration Assistant started logging out the user (10.9ish), but since then it has been a little more irritating to migrate from a dmg or other image format (as opposed to a source hard drive or time machine backup). Here is a simple Terminal command to mount the image as root and thus keep it mounted after logout.
sudo hdiutil attach diskimage.dmg
This process does require running a checksum, and for larger images that could take a while. Once you run this command and it completes its verification of the disk image, simply run Migration Assistant and complete the migration. After migration and a restart, the image is unmounted and you are ready to work on your newly migrated mac. (note: migration can also take quite a while depending on the amount of data)