Call for src twice, same client. Doesn't Work! Seems to use original snapshot of host workdir without exported build directory. To test ensure you've remove ./build/ before running.

main.go

package main

import (
    "context"
    "fmt"
    "os"
	"time"

    "dagger.io/dagger"
)

func main() {
    if err := build(context.Background()); err != nil {
        fmt.Println(err)
    }
}

func build(ctx context.Context) error {
    fmt.Println("Building with Dagger")

    // define build matrix
    oses := []string{"darwin"}
    arches := []string{"arm64"}
    goVersions := []string{"1.19"}

    // initialize Dagger client
    client, err := dagger.Connect(ctx, dagger.WithLogOutput(os.Stdout))
    if err != nil {
        return err
    }
    defer client.Close()

    // get reference to the local project
    src := client.Host().Workdir()

    // create empty directory to put build outputs
    outputs := client.Directory()

    for _, version := range goVersions {
        // get `golang` image for specified Go version
        imageTag := fmt.Sprintf("golang:%s", version)
        golang := client.Container().From(imageTag)
        // mount cloned repository into `golang` image
        golang = golang.WithMountedDirectory("/src", src).WithWorkdir("/src")

        for _, goos := range oses {
            for _, goarch := range arches {
                // create a directory for each os, arch and version
                path := fmt.Sprintf("build/%s/%s/%s/", version, goos, goarch)
                // set GOARCH and GOOS in the build environment
                build := golang.WithEnvVariable("GOOS", goos)
                build = build.WithEnvVariable("GOARCH", goarch)

                // build application
                build = build.Exec(dagger.ContainerExecOpts{
                    Args: []string{"go", "build", "-o", path},
                })

                // get reference to build output directory in container
                outputs = outputs.WithDirectory(path, build.Directory(path))
            }
        }
    }
    // write build artifacts to host
    _, err = outputs.Export(ctx, ".")
    // get reference to the local project again
    src2 := client.Host().Workdir()
	alpine := client.Container().From("alpine")
	alpine = alpine.WithMountedDirectory("/src", src2).WithWorkdir("/src")
	// ensure `ls` exec is never cached
	epoch := fmt.Sprintf("%d", time.Now().Unix())
	alpine = alpine.WithEnvVariable("CACHEBUSTER", epoch)
	out, err := alpine.Exec(dagger.ContainerExecOpts{
		Args: []string{"ls", "-alh", "build"},
	}).Stdout().Contents(ctx)
	fmt.Printf(out)
    if err != nil {
        return err
    }
    return nil
}