Docker "the --mount option requires BuildKit" - Enable BuildKit in CI

Your Dockerfile uses BuildKit-only syntax (RUN --mount=...) but the build ran with the legacy builder, which does not understand it.

What this error means

The build fails on the first RUN --mount=... line with the --mount option requires BuildKit. The same Dockerfile builds fine on a machine where BuildKit is the default.

docker build output

the --mount option requires BuildKit. Refer to https://docs.docker.com/go/buildkit/
to learn how to build images with BuildKit enabled.

Common causes

BuildKit is disabled on this runner

Older Docker, or an environment with DOCKER_BUILDKIT=0, falls back to the legacy builder. Cache/secret/ssh mounts are BuildKit features and are rejected.

Building through a path that bypasses BuildKit

Some tools or wrappers invoke the legacy build API even on a host that could use BuildKit, so the mount syntax is not recognized.

How to fix it

Enable BuildKit explicitly

Turn BuildKit on via the environment variable for the build.

Terminal

DOCKER_BUILDKIT=1 docker build .
# or set it for the whole job
export DOCKER_BUILDKIT=1

Use buildx for consistent BuildKit behavior

In GitHub Actions, set up buildx so BuildKit is always the builder.

.github/workflows/build.yml

- uses: docker/setup-buildx-action@v3
- uses: docker/build-push-action@v6
  with:
    context: .
    push: false

How to prevent it

Set DOCKER_BUILDKIT=1 (or use buildx) in any pipeline using mount syntax.
Pin a Docker version where BuildKit is the default.
Document the BuildKit requirement next to the Dockerfile.