Why does the order of instructions in a Dockerfile matter?

The order of instructions in a Dockerfile matters for three main reasons:

#Layer caching

Docker builds an image layer by layer and caches each step. If you change a line higher up in the Dockerfile, the cache for all the following layers is reset, and they are rebuilt from scratch. That is why commands that rarely change (for example, apt-get install or installing dependencies) are placed higher up, while the ones that change are placed lower (for example, copying source files).

#Build speed

The right order lets you use the cache and build the image much faster. If you break the cache every time because of the wrong instruction order, the build will take longer.

#Image size and cleanliness

A well-thought-out order lets you minimize the number of extra layers, and therefore reduce the final image size. For example, cleanup commands (rm, apt clean) are combined into a single RUN, so that no garbage is left in the previous layers.

Summary for junior: A Dockerfile is read and built top to bottom, and each step creates a layer. If you change the order of instructions thoughtlessly, the image will take longer to build, become larger, and the cache will stop working effectively.