build: add support for named contexts

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

build/build.go

@@ -14,6 +14,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"syscall"
 	"time"
 
 	"github.com/containerd/containerd/images"
@@ -79,6 +80,7 @@ type Inputs struct {
 	InStream         io.Reader
 	ContextState     *llb.State
 	DockerfileInline string
+	NamedContexts    map[string]string
 }
 
 type DriverInfo struct {
@@ -1080,6 +1082,27 @@ func LoadInputs(ctx context.Context, d driver.Driver, inp Inputs, pw progress.Wr
 
 	target.FrontendAttrs["filename"] = dockerfileName
 
+	for k, v := range inp.NamedContexts {
+		target.FrontendAttrs["frontend.caps"] = "moby.buildkit.frontend.contexts+forward"
+		if urlutil.IsGitURL(v) || urlutil.IsURL(v) || strings.HasPrefix(v, "docker-image://") {
+			target.FrontendAttrs["context:"+k] = v
+			continue
+		}
+		st, err := os.Stat(v)
+		if err != nil {
+			return nil, errors.Wrapf(err, "failed to get context %v", k)
+		}
+		if !st.IsDir() {
+			return nil, errors.Wrapf(syscall.ENOTDIR, "failed to get context path %v", v)
+		}
+		localName := k
+		if k == "context" || k == "dockerfile" {
+			localName = "_" + k // underscore to avoid collisions
+		}
+		target.LocalDirs[localName] = v
+		target.FrontendAttrs["context:"+k] = "local:" + localName
+	}
+
 	release := func() {
 		for _, dir := range toRemove {
 			os.RemoveAll(dir)

commands/build.go

@@ -20,6 +20,7 @@ import (
 	"github.com/docker/cli/cli"
 	"github.com/docker/cli/cli/command"
 	dockeropts "github.com/docker/cli/opts"
+	"github.com/docker/distribution/reference"
 	"github.com/docker/docker/pkg/ioutils"
 	"github.com/docker/go-units"
 	"github.com/moby/buildkit/client"
@@ -44,6 +45,7 @@ type buildOptions struct {
 	cacheFrom    []string
 	cacheTo      []string
 	cgroupParent string
+	contexts     []string
 	extraHosts   []string
 	imageIDFile  string
 	labels       []string
@@ -100,11 +102,17 @@ func runBuild(dockerCli command.Cli, in buildOptions) (err error) {
 		in.progress = "quiet"
 	}
 
+	contexts, err := parseContextNames(in.contexts)
+	if err != nil {
+		return err
+	}
+
 	opts := build.Options{
 		Inputs: build.Inputs{
 			ContextPath:    in.contextPath,
 			DockerfilePath: in.dockerfileName,
 			InStream:       os.Stdin,
+			NamedContexts:  contexts,
 		},
 		BuildArgs:   listToMap(in.buildArgs, true),
 		ExtraHosts:  in.extraHosts,
@@ -339,6 +347,8 @@ func buildCmd(dockerCli command.Cli, rootOpts *rootOptions) *cobra.Command {
 	flags.StringVar(&options.cgroupParent, "cgroup-parent", "", "Optional parent cgroup for the container")
 	flags.SetAnnotation("cgroup-parent", annotation.ExternalURL, []string{"https://docs.docker.com/engine/reference/commandline/build/#use-a-custom-parent-cgroup---cgroup-parent"})
 
+	flags.StringArrayVar(&options.contexts, "context", []string{}, "Additional named contexts (e.g., name=path)")
+
 	flags.StringVarP(&options.dockerfileName, "file", "f", "", `Name of the Dockerfile (default: "PATH/Dockerfile")`)
 	flags.SetAnnotation("file", annotation.ExternalURL, []string{"https://docs.docker.com/engine/reference/commandline/build/#specify-a-dockerfile--f"})
 
@@ -462,3 +472,23 @@ func listToMap(values []string, defaultEnv bool) map[string]string {
 	}
 	return result
 }
+
+func parseContextNames(values []string) (map[string]string, error) {
+	if len(values) == 0 {
+		return nil, nil
+	}
+	result := make(map[string]string, len(values))
+	for _, value := range values {
+		kv := strings.SplitN(value, "=", 2)
+		if len(kv) != 2 {
+			return nil, errors.Errorf("invalid context value: %s, expected key=value", value)
+		}
+		named, err := reference.ParseNormalizedNamed(kv[0])
+		if err != nil {
+			return nil, errors.Wrapf(err, "invalid context name %s", kv[0])
+		}
+		name := strings.TrimSuffix(reference.FamiliarString(named), ":latest")
+		result[name] = kv[1]
+	}
+	return result, nil
+}