Add entrypoint to PATH (ko-build#114)

This makes it easier to invoke the binary when using a debug container.
E.g. you could invoke `ko` instead of `/ko-app/ko`.

pkg/build/gobuild.go

@@ -21,6 +21,7 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"fmt"
 	gb "go/build"
 	"io"
 	"io/ioutil"
@@ -450,6 +451,7 @@ func (gb *gobuild) Build(ctx context.Context, s string) (v1.Image, error) {
 
 	cfg = cfg.DeepCopy()
 	cfg.Config.Entrypoint = []string{appPath}
+	updatePath(cfg, appPath)
 	cfg.Config.Env = append(cfg.Config.Env, "KO_DATA_PATH="+kodataRoot)
 	cfg.Author = "github.com/google/ko"
 
@@ -464,3 +466,24 @@ func (gb *gobuild) Build(ctx context.Context, s string) (v1.Image, error) {
 	}
 	return image, nil
 }
+
+// Append appPath to the PATH environment variable, if it exists. Otherwise,
+// set the PATH environment variable to appPath.
+func updatePath(cf *v1.ConfigFile, appPath string) {
+	for i, env := range cf.Config.Env {
+		parts := strings.SplitN(env, "=", 2)
+		if len(parts) != 2 {
+			// Expect environment variables to be in the form KEY=VALUE, so this is unexpected.
+			continue
+		}
+		key, value := parts[0], parts[1]
+		if key == "PATH" {
+			value = fmt.Sprintf("%s:%s", value, appPath)
+			cf.Config.Env[i] = "PATH=" + value
+			return
+		}
+	}
+
+	// If we get here, we never saw PATH.
+	cf.Config.Env = append(cf.Config.Env, "PATH="+appPath)
+}

pkg/build/gobuild_test.go

@@ -21,6 +21,7 @@ import (
 	"io/ioutil"
 	"path"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -336,7 +337,24 @@ func TestGoBuild(t *testing.T) {
 			}
 		}
 		if !found {
-			t.Error("Didn't find expected file in tarball.")
+			t.Error("Didn't find KO_DATA_PATH.")
+		}
+	})
+
+	// Check that PATH contains the produced binary.
+	t.Run("check PATH env var", func(t *testing.T) {
+		cfg, err := img.ConfigFile()
+		if err != nil {
+			t.Errorf("ConfigFile() = %v", err)
+		}
+		found := false
+		for _, entry := range cfg.Config.Env {
+			if strings.HasPrefix(entry, "PATH=") && strings.Contains(entry, "/ko-app/test") {
+				found = true
+			}
+		}
+		if !found {
+			t.Error("Didn't find entrypoint in PATH.")
 		}
 	})