What specific mechanism does Docker use during the image build process to speed up subsequent builds by reusing the results of unchanged steps from previous builds?

Docker uses a mechanism called layer caching to speed up subsequent image builds. A Docker image is composed of read-only layers, where each instruction in a Dockerfile creates a new layer. When Docker builds an image, it processes the Dockerfile instructions sequentially from top to bottom. For each instruction, Docker first checks if it has a cached layer that corresponds to that exact instruction from a previous build. This check involves comparing the instruction itself and, more importantly, a checksum of any files or data involved in that step. For example, for an instruction like `RUN apt-get update`, Docker computes a checksum of the command string. For `COPY source /destination`, Docker computes a checksum of the contents and metadata (like modification times) of the `source` files. If Docker finds a cached layer where both the instruction and its associated checksum are identical to the current step, it reuses that existing layer instead of executing the instruction again. This is known as a cache hit. The build process then skips the execution of that instruction and moves to the next one, utilizing the cached layer. If no matching cached layer is found, or if the instruction or its associated files have changed, Docker executes the instruction, creating a new layer. This is a cache miss. Once a cache miss occurs for any instruction, Docker automatically invalidates the cache for all subsequent instructions. This means all layers from that point onwards must be rebuilt, even if their instructions appear unchanged, because their parent layer (the one that missed the cache) is new. Therefore, to maximize cache reuse, frequently changing instructions, such as those that copy application code, are often placed towards the end of a Dockerfile, allowing earlier, more stable layers (like base operating system setup) to be consistently reused.