Can anyone tell me what this does? This looks like a configuration files in JSON, but why do these setting make it work in docker? If my docker image size is different, which parameter in this would I change?

Well this just tells the npm client to download the modules from official registry which is "https://registry.npmjs.org". Extra info: If you want even faster downloads within an organization(or wherever recurring npm install might occur), I would recommend setting up a local lazy mirror. PS: I maintain one such lazy npm mirror app-https://github.com/bhanuc/alphonso . It only stores the modules that has been requested and makes subsequent installs much faster.