Getting "$GOPATH/go.mod exists but should not" when running build in a Docker image

[David Karr]

I had earlier written a note about troubles refactoring my project to have a "src" directory, but I've backed off on that for now.  I see that the main problem I saw with that hasn't changed when backing out those changes. It appears that running this build inside a docker image seems to be somehow causing this problem.

In any case, I have a relatively simple go application with main.go, go.mod, and go.sum in the root directory, with subdirectories containing other source files in packages.

When I just run "go build ..." from the command line in the root directory of my project, it works fine and produces a working executable.

However, I'm trying to get this build to work within a published go "builder" image, which I will use as the first stage in a multi-stage Docker build.

I will first state that this has something to do with GOPATH, and I've gone through https://www.digitalocean.com/community/tutorials/understanding-the-gopath , but I'd have to say that I still don't understand its proper role here.

Not sure what you would want to see first, but here's a plain "ls" of the root directory of my project:

-------------------------

./         Dockerfile        go.mod           Jenkinsfile        README.md
../         .dockerignore        go.sum           lib/                target/
app/         .git/                handlers/  main.go        trustStore/
config/  .gitignore        include/   Makefile        funcs/

-----------------------

Just running "make build" does this:

---------------------

CGO_ENABLED=1 GOOS=linux GOARCH=amd64 go build -o ../target/dist/linux-amd64

---------------------

Which is fine.

My "dockerbuild" target in the Makefile is this:

--------------------

dockerbuild:
        docker build \
                --build-arg host_src_path=. \
                --build-arg packages=. \
                --build-arg executable_name=appgo \
                --build-arg cgo_enabled_01=1 \
                --build-arg goos=linux \
                --build-arg goarch=amd64 \
                -f Dockerfile -t appgo .

-----------------------

My Dockerfile, before the start of the second build stage, is this:

----------------------------

        FROM .../golang:1.17.6-bullseye as go-builder

        ARG packages
        ARG executable_name
        ARG host_src_path
        ARG cgo_enabled_01
        ARG goos
        ARG goarch

        COPY $host_src_path .
        RUN env
        RUN ls -lt
        RUN ls -lt $GOPATH
        #RUN go mod download

        RUN CGO_ENABLED=$cgo_enabled_01 GOOS=$goos GOARCH=$goarch go build -o $executable_name $packages

--------------------

When I run "make dockerbuild", I see this:

------------------

        Sending build context to Docker daemon  125.8MB
        Step 1/19 : FROM .../golang:1.17.6-bullseye as go-builder
         ---> 80d9a75ccb38
        Step 2/19 : ARG packages
         ---> Using cache
         ---> d98015e225b1
        Step 3/19 : ARG executable_name
         ---> Using cache
         ---> 1d336ac5cf1b
        Step 4/19 : ARG host_src_path
         ---> Using cache
         ---> 59595b09a376
        Step 5/19 : ARG cgo_enabled_01
         ---> Using cache
         ---> 3c4c08392ede
        Step 6/19 : ARG goos
         ---> Using cache
         ---> 6ed0d6881fb4
        Step 7/19 : ARG goarch
         ---> Using cache
         ---> 9e4b94e86b09
        Step 8/19 : COPY $host_src_path .
         ---> 4727e2fc3e80
        Step 9/19 : RUN env
         ---> Running in 8fcedaa86227
        HOSTNAME=8fcedaa86227
        HOME=/root
        packages=.
        cgo_enabled_01=1
        goarch=amd64
        host_src_path=.
        PATH=/go/bin:/usr/local/go/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
        goos=linux
        GOPATH=/go
        executable_name=appgo
        PWD=/go
        GOLANG_VERSION=1.17.6
        Removing intermediate container 8fcedaa86227
         ---> 46e44a5a3539
        Step 10/19 : RUN ls -lt
         ---> Running in 75107286d337
        total 128
        -rw-rw-r-- 1 root root   635 Feb 11 18:37 Dockerfile
        -rw-rw-r-- 1 root root   737 Feb 11 18:29 Makefile
        -rw-rw-r-- 1 root root   131 Feb 10 18:59 Jenkinsfile
        drwxrwxrwx 2 root root  4096 Jan 27 10:09 bin
        drwxrwxrwx 2 root root  4096 Jan 27 10:09 src
        -rw-rw-r-- 1 root root   967 Jan 13 18:34 main.go
        drwxrwxr-x 3 root root  4096 Jan 11 23:46 target
        drwxrwxr-x 2 root root  4096 Jan 11 23:46 funcs
        drwxrwxr-x 3 root root  4096 Jan 11 21:06 trustStore
        -rw-rw-r-- 1 root root  1454 Jan 11 21:06 README.md
        drwxrwxr-x 2 root root  4096 Jan 11 21:06 app
        drwxrwxr-x 2 root root  4096 Jan 11 21:06 config
        -rw-rw-r-- 1 root root   806 Jan 11 21:06 go.mod
        -rw-rw-r-- 1 root root 65194 Jan 11 21:06 go.sum
        drwxrwxr-x 2 root root  4096 Jan 11 21:06 handlers
        drwxrwxr-x 3 root root  4096 Jan 11 21:06 include
        drwxrwxr-x 4 root root  4096 Jan 11 21:06 lib
        Removing intermediate container 75107286d337
         ---> a880f28ef2b2
        Step 11/19 : RUN ls -lt $GOPATH
         ---> Running in 220d4e8dbb33
        total 128
        -rw-rw-r-- 1 root root   635 Feb 11 18:37 Dockerfile
        -rw-rw-r-- 1 root root   737 Feb 11 18:29 Makefile
        -rw-rw-r-- 1 root root   131 Feb 10 18:59 Jenkinsfile
        drwxrwxrwx 2 root root  4096 Jan 27 10:09 bin
        drwxrwxrwx 2 root root  4096 Jan 27 10:09 src
        -rw-rw-r-- 1 root root   967 Jan 13 18:34 main.go
        drwxrwxr-x 3 root root  4096 Jan 11 23:46 target
        drwxrwxr-x 2 root root  4096 Jan 11 23:46 funcs
        drwxrwxr-x 3 root root  4096 Jan 11 21:06 trustStore
        -rw-rw-r-- 1 root root  1454 Jan 11 21:06 README.md
        drwxrwxr-x 2 root root  4096 Jan 11 21:06 app
        drwxrwxr-x 2 root root  4096 Jan 11 21:06 config
        -rw-rw-r-- 1 root root   806 Jan 11 21:06 go.mod
        -rw-rw-r-- 1 root root 65194 Jan 11 21:06 go.sum
        drwxrwxr-x 2 root root  4096 Jan 11 21:06 handlers
        drwxrwxr-x 3 root root  4096 Jan 11 21:06 include
        drwxrwxr-x 4 root root  4096 Jan 11 21:06 lib
        Removing intermediate container 220d4e8dbb33
         ---> 76c8e73bc8e8
        Step 12/19 : RUN CGO_ENABLED=$cgo_enabled_01 GOOS=$goos GOARCH=$goarch go build -o $executable_name $packages
         ---> Running in 28a92ca3224d
        $GOPATH/go.mod exists but should not
        The command '/bin/sh -c CGO_ENABLED=$cgo_enabled_01 GOOS=$goos GOARCH=$goarch go build -o $executable_name $packages' returned a non-zero code: 1
        make: *** [Makefile:11: dockerbuild] Error 1

--------------------

There are a few things to note here, starting with the error message at the end:

------------------

$GOPATH/go.mod exists but should not

------------------

Outside of my Docker container, $GOPATH is basically ~/go, and it just contains directories "bin" and "pkg".  However, inside the container, $GOPATH has all the code for my project, including the "go.mod" file, so at least the first part of that error message is accurate, but I don't understand the significance of that, or what I'm supposed to do to resolve this.

[Sean Liao]

The `golang` container image sets the GOPATH directory to /go and the working directory to /go too.

Following GOPATH layout, source code is expected to be in directories under /go/src.

`/go/bin` is where binaries are installed by default, and `/go/pkg` include things like the module cache.

With modules, it is entirely possible to work outside of the GOPATH (as you most likely are doing).

In which case, a common pattern is to choose your own working directory,

eg `WORKDIR /workspace` and work in there.