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.

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