include example using go packages (#1145)

Signed-off-by: Jason Hall <[email protected]>

docs/advanced/go-packages.md

@@ -4,3 +4,58 @@
 
 To build an image, use [`pkg/build`](https://pkg.go.dev/github.com/google/ko/pkg/build), and publish it with [`pkg/publish`](https://pkg.go.dev/github.com/google/ko/pkg/publish).
 
+This is a minimal example of using the packages together, to implement the core subset of `ko`'s functionality:
+
+```go
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+
+	"github.com/google/go-containerregistry/pkg/authn"
+	"github.com/google/go-containerregistry/pkg/name"
+	"github.com/google/go-containerregistry/pkg/v1/remote"
+	"github.com/google/ko/pkg/build"
+	"github.com/google/ko/pkg/publish"
+)
+
+const (
+	baseImage  = "cgr.dev/chainguard/static:latest"
+	targetRepo = "example.registry/my-repo"
+	importpath = "github.com/my-org/miniko"
+	commitSHA  = "deadbeef"
+)
+
+func main() {
+	ctx := context.Background()
+
+	b, err := build.NewGo(ctx, ".",
+		build.WithPlatforms("linux/amd64"), // only build for these platforms.
+		build.WithBaseImages(func(ctx context.Context, _ string) (name.Reference, build.Result, error) {
+			ref := name.MustParseReference(baseImage)
+			base, err := remote.Index(ref, remote.WithContext(ctx))
+			return ref, base, err
+		}))
+	if err != nil {
+		log.Fatalf("NewGo: %v", err)
+	}
+	r, err := b.Build(ctx, importpath)
+	if err != nil {
+		log.Fatalf("Build: %v", err)
+	}
+
+	p, err := publish.NewDefault(targetRepo,                 // publish to example.registry/my-repo
+		publish.WithTags([]string{commitSHA}),               // tag with :deadbeef
+		publish.WithAuthFromKeychain(authn.DefaultKeychain)) // use credentials from ~/.docker/config.json
+	if err != nil {
+		log.Fatalf("NewDefault: %v", err)
+	}
+	ref, err := p.Publish(ctx, r, importpath)
+	if err != nil {
+		log.Fatalf("Publish: %v", err)
+	}
+	fmt.Println(ref.String())
+}
+```