I too have been struggling with Docker, until that is, that someone asked me to create 100 mini moodles for schools across Lancashire.
My first thoughts were very small Azure VMs with shared resources that cost very little. But then I realised after reading about docker, that i could build a mini moodle, dockerise it and then have 5 medium VMs in Azure and have 20 mini moodles containerised on each VM
After I have built one mini Moodle, that worked and was easily configurable, I just created the rest in quick succession and Roberts your fathers brother