docs(BestPractices.md): add dependency caching guide

[yxliang01]

To avoid performing npm install again and again when any source code in the project is being modified, it's (always?) preferred to cached node_modules as long as package.json and package-lock.json are not changed. I think this can be added to the BestPractices.md guide. Welcome for discussions on whether this is really a good practice also.

[yosifkit]

That is what the deprecated onbuild images would do. Perhaps it needs to be dropped into the best practices doc (minus the ONBUILD keyword)?

Or just update the Dockerfile in the how-to-use this image section, since that two line Dockerfile doesn't really containerize an app unless it has a COPY.