RUN --mount=type=cache causes the whole build context to be loaded in

[hach-que]

We're using buildkit because it drastically reduces the size of the build context that needs to be copied across during builds. However, it looks like RUN --mount=type=cache causes the entire build context to be copied for some reason.

Before:

COPY something /something
WORKDIR /something/
RUN go build

=> [internal] load build context                                                                                                                                                 0.1s  
=> => transferring context: 1.27MB                                                                                                                                               0.0s

After:

COPY something /something
WORKDIR /something/
RUN --mount=type=cache,target=/.cache go build

=> [internal] load build context                                                                                                                                               103.3s  
=> => transferring context: 5.28GB                                                                                                                                             103.3s
...

This is on Windows 10 with experimental features enabled in Docker.

[hach-que]

Is there a timeline / ETA for when this fix will be in Docker for Windows? Is there a way we can use a fixed version with Docker ahead of it's release there?

Right now buildkit is something that is turned on with an environment variable DOCKER_BUILDKIT=1, but I have no idea how to point Docker at a different version.

[AkihiroSuda]

With docker buildx you can always use the latest BuildKit with an old Docker daemon

[tonistiigi]

@hach-que You can start using/testing this fix by switching the syntax line to #syntax=docker/dockerfile-upstream:master-experimental. That will work with any docker version that has buildkit support. We'll probably do a new versioned release this week.

[hach-que]

Oh, that's super convenient. I had no idea it worked like that.

[xkszltl]

@hach-que You can start using/testing this fix by switching the syntax line to #syntax=docker/dockerfile-upstream:master-experimental. That will work with any docker version that has buildkit support. We'll probably do a new versioned release this week.

@tonistiigi
I tried the upstream syntax docker with docker-ce 18.09.8 and 19.03 (rc) but failed.
18.09.8 says exit code: 2 and 19.03 says frontend grpc server closed unexpectedly.

Screenshot of 18.09.8

Screenshot of 19.03 rc

[xkszltl]

I even tried the docker-ce nightly (2019-07-19), and that seems working, but not always.
There were several times where I got failed to solve with frontend dockerfile.v0: failed to solve with frontend gateway.v0: frontend grpc server closed unexpectedly

[tonistiigi]

@xkszltl Please post daemon logs. Feel free to create a new issue as it doesn't look your issue is related to the original post.

[xkszltl]

@tonistiigi I opened #1090 to track this

[thaJeztah]

fix was cherry-picked in Docker 19.03 through docker-archive/engine#312