When working in a containers based environment, it’s very frequent to build Docker images that are agnostic to the environment. That way, they can be deployed in any environment, from local development to production, which gives the great advantage of preventing any change in a Docker image taht was certified internally in a testing/staging environment for deploying it into production.

However, this may create an issue while developing, where you want to test your changes as soon as possible, hence it’s convenient to skip creating a new Docker image every time a change in the source code is detected.

A common pattern is to run locally a Docker container mounting the folder where your source code is located into the folder where it’s included while generating the Docker image (assuming there’s no compilation / packaging / etc for going straight into the point :) ).

For example, let’s assume a Node.js application that keeps the source code in the root folder of the repository in the host machine and that copies it into the /usr/src/app folder of the Docker image. Mounting the source code folder can be accomplished with the following command:

docker run -v $(PWD):/usr/src/app my-app:latest

While this is very straight forward, it may create a big issue, as you don’t want, at all, to mount your node_modules subfolder into the Docker container. Some dependencies might require native code/compilation, and if your host machine does not match the one used in the Docker image, you’d get into troubles.

To prevent this, a special volume option pointing to the node_modules subfolder can be included, and this folder won’t be overwritten by the host information:

docker run \
  -v $(PWD):/usr/src/app \
  -v /usr/src/app/node_modules \
  my-app:latest

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