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If your CI Docker builds repeatedly download dependencies or recompile unchanged code, use BuildKit cache mounts for those tool caches and separately configure BuildKit layer-cache import and export. The “15 minutes” in the original headline is a reader scenario, not a documented typical build time or measured saving. One important catch: GitHub Actions’ BuildKit cache does not preserve cache-mount contents by default, so ephemeral runners need an additional strategy.
What BuildKit cache mounts do—and what they do not
A cache mount makes a directory available to a Dockerfile RUN instruction. Package managers and compilers can reuse files there on a later build, avoiding some repeated downloads or compilation. For example, Docker documents a Go build-cache mount at /root/.cache/go-build.
A cache mount is not the same as Docker’s ordinary instruction or layer cache. Layer caching can reuse the result of a Dockerfile instruction; a cache mount supplies reusable tool data while an instruction runs. Configure both when your build needs both kinds of reuse. Docker’s rule is explicit: “Cache mounts should only be used for better performance.” A build must remain correct if the mounted directory is empty, replaced, or removed by garbage collection. Docker’s Dockerfile reference
First determine whether your builder survives between CI runs
Persistent builder
When builds use the same persistent BuildKit builder, mount data can persist between invocations. It is still disposable: BuildKit may overwrite or garbage-collect it, so correctness cannot depend on it.
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Ephemeral GitHub-hosted runner
A fresh runner generally cannot rely on mount contents from a previous job. Exporting ordinary BuildKit cache to GitHub Actions does not automatically save the directories used by RUN --mount=type=cache. Docker documents reproducible-containers/buildkit-cache-dance as a workaround that extracts and injects mount data around the build. Its documentation example pins the action to commit 4b2444fec0c0fb9dbf175a96c094720a692ef810 and labels it v2.1.4; check the upstream project for current configuration before adopting it. Docker’s GitHub Actions cache-management guide
Add mounts for the expensive tool caches
Place a mount on the instruction that runs the package manager or compiler, and choose a target directory that the tool actually uses. Docker’s documented Go example mounts /root/.cache/go-build; its GitHub Actions cache-management example also mounts Go modules at /go/pkg/mod. A mount does not help if the relevant command writes its cache somewhere else.
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Here is the basic syntax, adapted to a Go build:
# syntax=docker/dockerfile:1
FROM golang:1.24 AS build
WORKDIR /src
COPY go.mod go.sum ./
RUN --mount=type=cache,target=/go/pkg/mod
go mod download
COPY . .
RUN --mount=type=cache,target=/root/.cache/go-build
go build -o /out/app .The Go image version and example are illustrative; select a base image and build command appropriate to your project. The important configuration is the mount on the command that populates or consumes the cache. Since the mount is only an optimization, the commands must also work after its contents are lost.
Choose an ID and sharing mode deliberately
The mount ID defaults to its target path. Set an explicit id when you need to separate caches that otherwise share a target. BuildKit supports three sharing modes:
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shared: concurrent builds may use the same cache.private: concurrent builds get separate cache instances.locked: another build waits until the current user releases the cache.
Choose based on the tool’s concurrency behavior. Docker’s apt example uses sharing=locked because apt needs exclusive access to its data. Docker’s Dockerfile reference
Cache apt downloads safely
Docker’s apt example disables apt’s cleanup configuration so downloaded packages remain available, then mounts both apt data directories with locked sharing:
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# syntax=docker/dockerfile:1
FROM ubuntu:latest
RUN rm -f /etc/apt/apt.conf.d/docker-clean
RUN --mount=type=cache,target=/var/cache/apt,sharing=locked
--mount=type=cache,target=/var/lib/apt,sharing=locked
apt-get update && apt-get install -y --no-install-recommends curlThis reflects Docker’s documented pattern, not a universal apt configuration. Keep package-index updates and installation together in the same instruction, and adapt package names and base image to your needs. Docker’s Dockerfile reference
Export ordinary BuildKit cache separately
Cache mounts retain tool-specific directories; cache-from and cache-to import and export BuildKit’s reusable build cache. On GitHub Actions, Docker documents this pattern:
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- name: Build and push
uses: docker/build-push-action@v6
with:
context: .
push: true
tags: user/app:latest
cache-from: type=gha
cache-to: type=gha,mode=maxThe action version here is an example, not a claim that it is the newest release. Check current Docker documentation for action and backend requirements. The GitHub Actions cache backend is experimental, subject to GitHub’s size and usage limits, and depends on the Buildx driver. With the default docker driver, Docker says the containerd image store must be enabled. Docker’s GitHub Actions cache backend documentation
Docker also documents registry-backed cache export using a separate cache reference and mode=max. Inline cache is simpler but supports only min mode. Choose the backend based on where the builder can read and write cache and what its limits allow; neither approach should be mistaken for automatic cache-mount persistence. Docker’s cache-management guide Docker’s Buildx build reference
Handle read-only GitHub workflows without weakening permissions
Some GitHub events, including issue_comment and pull_request_target in the default-branch context, have read-only access to the Actions cache by default. A workflow may therefore import a cache successfully but fail when it tries to export one. For such a workflow, keep cache-from if useful and omit cache-to; populate the cache in a workflow with write access, such as a push workflow on the default branch.
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Do not grant cache-write access broadly to untrusted workflows just to make exports succeed: Docker warns this creates cache-poisoning risk. Docker’s workflow-permissions guidance
Choose a caching approach for your CI setup
| Builder and need | Approach | What it preserves |
|---|---|---|
| Persistent builder; tool cache needed | Use RUN --mount=type=cache with suitable targets and sharing. |
Mount data may persist between invocations, but can be removed or overwritten. |
| Ephemeral runner; reusable build results needed | Configure BuildKit cache import/export, such as GitHub Actions or registry cache. | Exported BuildKit cache; this alone does not preserve cache-mount directories. |
| Ephemeral runner; tool cache-mount data needed too | Use a documented extraction/injection workaround such as buildkit-cache-dance, or adopt a persistent-builder strategy. | Mount data through the additional workaround or builder persistence; verify current implementation details. |
These choices can be combined: a common CI setup exports ordinary BuildKit cache and separately handles mount directories when builds run on fresh runners. Before implementing, verify current GitHub cache limits, Buildx driver requirements, workflow permissions, and version-sensitive setup in Docker’s documentation—especially for self-managed runners, where requirements can differ or change.
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