I was in the process of writing basically this same post, but you’ve already said it very well.
Docker isn’t really a technology in and of itself; its an applicable of many of the fundamental components of Linux to create a specific result. To really understand it you have to understand the technologies it’s built from.
A recommendation I’ll throw in here for OP is How Linux Works by Brian Ward. This book is an excellent first step in really understanding those Linux fundamentals.