Reuse cache mounts reliably across multi-stage Dockerfiles
Problem summary When a small source change makes a Docker build re-run dependency installs, many developers say “the cache didn’t work.” The clearer diagnosis is that Docker has at least two different cache mechanisms: instruction/layer cache (which can skip an instruction entirely) and cache mounts (which provide a persistent directory to a step when that step runs). They solve different…
When a small change in a Dockerfile triggers a re-build of dependencies, many developers mistakenly believe their cache is not functioning. The true cause is often Docker's dual caching mechanisms: instruction/layer cache (which can entirely bypass an instruction) and cache mounts (which offer a persistent directory for a step during execution).
These two mechanisms serve distinct purposes and possess varying guarantees. The following guide focuses on creating a helper that validates and formats Docker's cache mount declarations as documented, ensuring stable and readable identities for multi-stage builds.
The helper function `validateCacheMountInput` checks that the mount target is a non-empty absolute path made up of safe characters, that an optional id is also a non-empty safe string if provided, and that the sharing mode is one of the three documented values: shared, private, or locked. The helper then returns the correctly structured `--mount` fragment for inclusion in a Dockerfile instruction.
By default, the `id` is set to the `target` path, aligning with Docker's documented default and providing a stable, readable identifier in multi-stage builds. When using the `locked` sharing mode, it's appropriate for tools that require exclusive access to the cache files, enhancing file-locking semantics in CI environments with concurrent builds.
However, it is important to note that a correctly declared mount does not guarantee persistence across fresh builder instances. Docker stores cache mounts in the builder's local storage, and if your CI creates a new builder for each job or the builder clears storage, the mount will begin empty. While cache mounts can significantly speed up reruns by preserving downloaded artifacts, they do not alter Docker's invalidation rules.
If a dependency manifest changes, the install step should be rerun; the mount simply reduces the download cost during that rerun.
The helper function `buildCacheMountOptions` validates the input and returns the appropriate `--mount` options. The `formatRunMount` function formats the options into a `--mount` string ready for inclusion in a Dockerfile. By separating the guaranteed functionality of the Dockerfile from the environmental requirements (builder-local persistence), developers can reason about performance more predictably: first ensure the mount declaration is correct, then address any persistence issues in your CI or builder topology.
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