I had a slightly similar issue with a fresh install, it wasn’t getting as far as the preparing part and was just flat out saying it couldn;t find docker at all. Turns out I was trying to use a 5v 1a PSU, when I plugged in a slightly more powerful Amazon 9W charger which I think it 5v 1.8a it has powered up with no issues at all. Now i’m going to buy an approved Pi3 charge which is 5v 2.5a so that I can put any power concerns to bed!