What's the purpose of the node-modules container in wolkenkit?

When starting a wolkenkit application, the application is boxed in a number of Docker containers, and these containers are then started along with a few other containers that provide the infrastructure, such as databases, a message queue, ...

The reason why the application is split into several Docker containers is because wolkenkit builds upon the CQRS pattern, which suggests separating the read side of an application from the application's write side, and hence there is one container for the read side, and one for the write side (actually there are a few more, but you get the picture).

Now, since you may develop on an operating system other than Linux, the wolkenkit application may run under a different operating system than when you develop it, as within Docker it's always Linux. This means that the start command can not simply copy over the node_modules folder into the containers, as they may contain binary modules, which are then not compatible (imagine installing on Windows on the host, but running on Linux within Docker).

To avoid issues here, wolkenkit runs an npm install when starting the application inside of the containers. The problem now is that if wolkenkit did this in every single container, the start would be super slow (it's not the fastest thing on earth anyway, due to all the Docker building and starting that's happening under the hood). So wolkenkit tries to optimize this as much as possible.

One concept here is to run npm install only once, inside of a container of its own. This is the node-modules container you encountered. This container is then linked as a volume to all the containers that contain the application's code. This way you only have to run npm install once, but multiple containers can use the outcome of this command.

Since this container now contains data, but no code, it only has to be there, it doesn't actually do anything. This is why it gets created, but is not run.

I hope this makes it a little bit clearer, and I was able to answer your question :-)

PS: Please note that I am one of the core developers of wolkenkit, so take my answer with a grain of salt.