How do BuildKit cache mounts differ from regular Docker layer caching?

Regular layer caching reuses an entire previous layer if its instruction and inputs are unchanged. A BuildKit cache mount (--mount=type=cache,target=/root/.m2) instead persists a specific directory's contents (like a package manager's download cache) across builds without that directory ever becoming part of any image layer — useful because dependency download caches can be reused even when the layer that would have contained them does need to rebuild (e.g., because the source code COPY changed in the same RUN step).