Express is a microframework that according to 2018 Node.js User Survey Report is used by 4 in 5 back-end and full-stack node.js developers. Thanks to its simplicity, the always-growing range of available middleware, and active community the express userbase is still growing.
Docker is the response to an ongoing problem of differences between environments in which application runs. Whether those differences are across machines of the development team, continuous integration server, or production environment. Since you are reading this, I assume you are already more or less familiar with benefits of containerizing applications. Let’s go straight to Node.js specific bits.