During development or with any image, which you need to update rather often, you usually don't want to lose all of docker's caching by putting everything into one giant RUN directive. This is one, where early optimization strikes hard. Don't merge RUN directives from the start. First build your image in a non-optimized way, saving loads of build time making use of docker build cache.

Personally I would not merge steps, which have nothing to do with each other, unless I am sure, that they are basically set in stone forever.

With public and widely popular base images, which are not changed once they are released, the choices might be weighed differently, as all the people, who make use of your image, will want to have fast download and small resulting images building on top of it.

Simply put: Don't make your development more annoying than necessary, by unnecessarily introducing long wait times for building docker images.