Move info about changing the location of docker images before creating the image

Yes, I made this stupid mistake of not reading "Troubleshooting" before creating an image and got quite upset when I realized it took a lot of space of my system partition, because it's quite small and the new changes in Linux Mint policy and app manager made me already lose around 3-4 GB of space (10 + 4 = 14, which is nearly half of 30 GB which I gave the system).

Of course it doesn't mean that the instruction is wrong - I made this mistake of not reading to the end before I started to do it (I'm sure I read it earlier, but not just before trying) - but still, I think moving it up in the instruction might help others from making the same mistake.