Docker cache mounts deletes downloaded dependencies

General
,
1

Hi guys.
I have a small problem related to docker cache mount.
My application written in golang and i want to speed up my image builds and because i decide to use docker cache mounts to cache the $GOPATH/pkg/mod and $USER/.cache/go-build
for speed up go mod download and go build commands
but cache mount didnt work correctly when i run this command in image building state

RUN --mount=type=cache,target="/go/pkg/mod" go mod download -x

after that i check the size of pkg/mod folder size and it is kilobytes
and when i run the command for building

RUN --mount=type=cache,target="/root/.cache/go-build" go build -o app

it downloades the dependencies again and i am sure that cache mount didnt work correctly if anybody can help me please react to my message.
Thank you.

2

What is your expectation? You use two different folders as target. That is two different caches, so unless you mount the cache of “/go/pkg/mod” in the second instruction, it will not be mounted again.

The context of your instructions is missing too. I guess you have a multistage build, right? Otherwise you wouldn’t need a cache the filesystem is the only thing that is kept between the instructions and you just save data in the cache which you don’t use in the second instruction.

Update:

I got confused as well. What I wrote is true, but something doesn’t seem to work for me either.

Update 2:

This is my cache test and seems working, but I added --no-cache to the build command as usually when I test something, so of course, it didn’t work:

# syntax=docker/dockerfile:1

FROM ubuntu:22.04 as base

RUN --mount=type=cache,target=/cache \
   echo "base" > /cache/t0.txt \
 && mkdir /copy

FROM base as build-client

RUN --mount=type=cache,target=/cache \
    echo "build-client" >> /cache/t1.txt \
 && cp /cache/t1.txt /copy/

FROM base as build-server

RUN --mount=type=cache,target=/cache \
    echo "build-server" >> /cache/t2.txt \
 && cp /cache/t1.txt /copy/t1-2.txt \
 && cp /cache/t2.txt /copy

FROM scratch as client
COPY --from=build-client /copy /

FROM scratch as server
COPY --from=build-server /copy /

Then run

docker build . -t localhost/client --progress plain --target client --output .
docker build . -t localhost/server --progress plain --target server --output .
ls

Output:

Dockerfile t1-2.txt  t1.txt  t2.txt

t1-2.txt wouldn’t be there without the cache as it is the copy of t1.txt which was not there i the base image, only in the cache.