@@ -96,6 +96,11 @@ type Builder struct {

 	ForceRemove bool

 	Pull        bool

+	// set this to true if we want the builder to not commit between steps.

+	// This is useful when we only want to use the evaluator table to generate

+	// the final configs of the Dockerfile but dont want the layers

+	disableCommit bool

+

 	AuthConfig     *registry.AuthConfig

 	AuthConfigFile *registry.ConfigFile

@@ -60,6 +60,9 @@ func (b *Builder) readContext(context io.Reader) error {

 }

 func (b *Builder) commit(id string, autoCmd []string, comment string) error {

+	if b.disableCommit {

+		return nil

+	}

 	if b.image == "" && !b.noBaseImage {

 		return fmt.Errorf("Please provide a source image with `from` prior to commit")

 	}

@@ -1,12 +1,16 @@

 package builder

 import (

+	"bytes"

+	"encoding/json"

+	"fmt"

 	"io"

 	"io/ioutil"

 	"os"

 	"os/exec"

 	"github.com/docker/docker/api"

+	"github.com/docker/docker/builder/parser"

 	"github.com/docker/docker/daemon"

 	"github.com/docker/docker/engine"

 	"github.com/docker/docker/graph"

@@ -14,6 +18,7 @@ import (

 	"github.com/docker/docker/pkg/parsers"

 	"github.com/docker/docker/pkg/urlutil"

 	"github.com/docker/docker/registry"

+	"github.com/docker/docker/runconfig"

 	"github.com/docker/docker/utils"

 )

@@ -24,6 +29,7 @@ type BuilderJob struct {

 func (b *BuilderJob) Install() {

 	b.Engine.Register("build", b.CmdBuild)

+	b.Engine.Register("build_config", b.CmdBuildConfig)

 }

 func (b *BuilderJob) CmdBuild(job *engine.Job) engine.Status {

@@ -138,3 +144,58 @@ func (b *BuilderJob) CmdBuild(job *engine.Job) engine.Status {

 	}

 	return engine.StatusOK

 }

+

+func (b *BuilderJob) CmdBuildConfig(job *engine.Job) engine.Status {

+	if len(job.Args) != 0 {

+		return job.Errorf("Usage: %s\n", job.Name)

+	}

+	var (

+		validCmd = map[string]struct{}{

+			"entrypoint": {},

+			"cmd":        {},

+			"user":       {},

+			"workdir":    {},

+			"env":        {},

+			"volume":     {},

+			"expose":     {},

+			"onbuild":    {},

+		}

+

+		changes   = job.Getenv("changes")

+		newConfig runconfig.Config

+	)

+

+	if err := job.GetenvJson("config", &newConfig); err != nil {

+		return job.Error(err)

+	}

+

+	ast, err := parser.Parse(bytes.NewBufferString(changes))

+	if err != nil {

+		return job.Error(err)

+	}

+

+	builder := &Builder{

+		Daemon:        b.Daemon,

+		Engine:        b.Engine,

+		Config:        &newConfig,

+		OutStream:     ioutil.Discard,

+		ErrStream:     ioutil.Discard,

+		disableCommit: true,

+	}

+

+	for i, n := range ast.Children {

+		cmd := n.Value

+		if _, ok := validCmd[cmd]; ok {

+			if err := builder.dispatch(i, n); err != nil {

+				return job.Error(err)

+			}

+		} else {

+			fmt.Fprintf(builder.ErrStream, "# Skipping serialization of instruction %s\n", strings.ToUpper(cmd))

+		}

+	}

+

+	if err := json.NewEncoder(job.Stdout).Encode(builder.Config); err != nil {

+		return job.Error(err)

+	}

+	return engine.StatusOK

+}