how to create a dmg for a mac from linux

You need first to create a dmg and then copy the files inside.

The following creates a dmg file (128M) that can be mounted on the mac. 

sudo apt install hfsprogs -y dd if=/dev/zero of=/tmp/dmgtmp.dmg bs=1M count=128 mkfs.hfsplus -v ThisIsADmg /tmp/dmgtmp.dmg

Then you can mount in linux with something like

mount -o loop /tmp/dmgtmp.dmg /mnt/dmgtmp

then copy the content in it (in /mnt/dmgtmp). 

After that you can can copy the dmg on a mac and mount it there.

:) my two cents Alex