Its often useful to run a command before Railway builds a Docker image. An immediate use case is to initialise and fetch Git submodules. See https://station.railway.com/questions/building-an-image-with-git-submodules-014a51ab. Generally, this is to "prepare a source tree" before it can be built. C/C++/"old native" codebases are like this.

The command has to be run right after you pull the source and before you start building the image.