I am unable to reproduce this behavior. Here are the steps I followed. Is there anything different about your execution?

$ docker run -it golang:1.13.0
/go# mkdir -p src/app/cmd
/go# cat > src/app/cmd/filename.go
package main

import (
"log"
"os/exec"
"path/filepath"
"runtime"
"os"
)

func main() {
binPath := runtime.GOROOT()
binPath = filepath.Join(binPath, "bin", "go")

srcPath := os.Getenv("GOPATH")
srcPath = filepath.Join(srcPath, "src", "app", "cmd", "filename.go")

cmd := exec.Command(binPath, "fmt", srcPath)
if err := cmd.Start(); err != nil {
    log.Println(err)
}
if err := cmd.Wait(); err != nil {
log.Println(err)
}
}
^C
/go# go run src/app/cmd/filename.go
/go# cat src/app/cmd/filename.go
package main

import (
    "log"
    "os"
    "os/exec"
    "path/filepath"
    "runtime"
)

func main() {
    binPath := runtime.GOROOT()
    binPath = filepath.Join(binPath, "bin", "go")

    srcPath := os.Getenv("GOPATH")
    srcPath = filepath.Join(srcPath, "src", "app", "cmd", "filename.go")

    cmd := exec.Command(binPath, "fmt", srcPath)
    if err := cmd.Start(); err != nil {
        log.Println(err)
    }
    if err := cmd.Wait(); err != nil {
        log.Println(err)
    }
}

@smasher164

Thank you for your interest in this issue.
There was a lack of basic information.

I ran it the same way as you and it worked fine.
When I ran the docker image with the host volume mounted, a failure occurred, like this.
docker run -it --rm -v d:\gopath\src\app:/go/src/app golang:1.13.0

I use docker desktop community version 2.1.0.3(38240)
OS : windows 10 version 1709(OS build 16299.1087)

It failed with ci runner docker in gitlab running as a self-host, too.
gitlab ci logs are like this.

Running with gitlab-runner 11.5.0 (3afdaba6)
on Shared.Linux.Docker.01 fc8c0bfa
Using Docker executor with image registry-gitlab-host/group/project/golang:1.13.0 ...
Pulling docker image registry-gitlab-host/group/project/golang:1.13.0 ...
Using docker image sha256:ae999aee95607f18ff924f57cc334fe2089903398ae9a0c9480a38d8b3c7fe5f for registry-gitlab-host/group/project/golang:1.13.0 ...
Running on runner-fc8c0bfa-project-1369-concurrent-0 via gitce-runl01.nxdmz.net...

Under the same conditions(on docker images with volume mounted / gitlab-ci worker on docker),
until golang version 1.12.9 worked fine.

Thank you.

I'm going to try reproducing this on a windows server installation. Will report back with details.

@smasher164

Thank you so much.
I found the reason.

It's my mistake, so sorry.
Your example works fine with or without the docker volume mounted also.

In my original case, I was vendoring and storing a module in a private repository,
but go modules tried to access it, even if I set go env GOPRIVATE, and it failed.
So the child process was dying.

Thank you.

I wrote some notes with additional informations.

if it’s case like that(using vendor option with a private repository),
I should run go commands with -mod=vendor options.

ex) go build -mod=vendor [target]

Thanks.