add feature to export to docker tarball (#140)

* add feature to export to docker tarball

* throw error if multiple platforms specified for tar export

builder/builder.go

@@ -398,7 +398,7 @@ func (h *HeighlinerBuilder) buildChainNodeDockerImage(
 			buildKitOptions.Platform = buildCfg.Platform
 		}
 		buildKitOptions.NoCache = buildCfg.NoCache
-		if err := docker.BuildDockerImageWithBuildKit(ctx, reldir, imageTags, push, buildArgs, buildKitOptions); err != nil {
+		if err := docker.BuildDockerImageWithBuildKit(ctx, reldir, imageTags, push, buildCfg.TarExportPath, buildArgs, buildKitOptions); err != nil {
 			return err
 		}
 	} else {

builder/types.go

@@ -49,6 +49,7 @@ type ChainNodeDockerBuildConfig struct {
 type HeighlinerDockerBuildConfig struct {
 	ContainerRegistry string
 	SkipPush          bool
+	TarExportPath     string
 	UseBuildKit       bool
 	BuildKitAddr      string
 	Platform          string

cmd/build.go

@@ -54,6 +54,7 @@ const (
 	flagNumber       = "number"
 	flagParallel     = "parallel"
 	flagSkip         = "skip"
+	flagTarExport    = "tar-export-path"
 	flagLatest       = "latest"
 	flagLocal        = "local"
 	flagUseBuildkit  = "use-buildkit"
@@ -158,6 +159,7 @@ An optional flag --tag/-t is now available to override the resulting docker imag
 	// Docker specific flags
 	buildCmd.PersistentFlags().StringVarP(&buildConfig.ContainerRegistry, flagRegistry, "r", "", "Docker Container Registry for pushing images")
 	buildCmd.PersistentFlags().BoolVarP(&buildConfig.SkipPush, flagSkip, "s", false, "Skip pushing images to registry")
+	buildCmd.PersistentFlags().StringVar(&buildConfig.TarExportPath, flagTarExport, "", "File path to export built image as docker tarball")
 	buildCmd.PersistentFlags().BoolVarP(&buildConfig.UseBuildKit, flagUseBuildkit, "b", false, "Use buildkit to build multi-arch images")
 	buildCmd.PersistentFlags().StringVar(&buildConfig.BuildKitAddr, flagBuildkitAddr, docker.BuildKitSock, "Address of the buildkit socket, can be unix, tcp, ssl")
 	buildCmd.PersistentFlags().StringVarP(&buildConfig.Platform, flagPlatform, "p", docker.DefaultPlatforms, "Platforms to build (only applies to buildkit builds with -b)")

docker/buildkit.go

@@ -1,8 +1,10 @@
 package docker
 
 import (
+	"bufio"
 	"context"
 	"fmt"
+	"io"
 	"os"
 	"strings"
 
@@ -37,11 +39,24 @@ func GetDefaultBuildKitOptions() BuildKitOptions {
 	}
 }
 
+type WriteCloser struct {
+	f *os.File
+	*bufio.Writer
+}
+
+func (wc *WriteCloser) Close() error {
+	if err := wc.Flush(); err != nil {
+		return err
+	}
+	return wc.f.Close()
+}
+
 func BuildDockerImageWithBuildKit(
 	ctx context.Context,
 	dockerfileDir string,
 	tags []string,
 	push bool,
+	tarExport string,
 	args map[string]string,
 	buildKitOptions BuildKitOptions,
 ) error {
@@ -54,16 +69,36 @@ func BuildDockerImageWithBuildKit(
 
 	eg, ctx := errgroup.WithContext(ctx)
 
-	export := client.ExportEntry{
-		Type: "image",
-		Attrs: map[string]string{
-			"name": strings.Join(tags, ","),
-		},
-	}
-	if push {
-		export.Attrs["push"] = "true"
+	exports := make([]client.ExportEntry, 1)
+
+	if tarExport != "" {
+		if len(strings.Split(buildKitOptions.Platform, ",")) > 1 {
+			return fmt.Errorf("when using tar-export-path, only one platform is supported")
+		}
+
+		exports[0] = client.ExportEntry{
+			Type: "docker",
+			Output: func(m map[string]string) (io.WriteCloser, error) {
+				f, err := os.Create(tarExport)
+				if err != nil {
+					return nil, err
+				}
+
+				return &WriteCloser{f, bufio.NewWriter(f)}, nil
+			},
+		}
+	} else {
+		export := client.ExportEntry{
+			Type: "image",
+			Attrs: map[string]string{
+				"name": strings.Join(tags, ","),
+			},
+		}
+		if push {
+			export.Attrs["push"] = "true"
+		}
+		exports[0] = export
 	}
-	exports := []client.ExportEntry{export}
 
 	opts := map[string]string{
 		"platform": buildKitOptions.Platform,