Add platform flag to build (#217)

README.md

@@ -236,6 +236,7 @@ Flags:
   --label        Set metadata for an image (default: [])
   --no-cache     Do not use cache when building the image (default: false)
   --no-console   Use non-console progress UI (default: false)
+  --platform     Set platforms for which the image should be built (default: <yourPlatform>)
   -s, --state    directory to hold the global state (default: /home/user/.local/share/img)
   -t, --tag      Name and optionally a tag in the 'name:tag' format (default: [])
   --target       Set the target build stage to build (default: <none>)

build.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"github.com/containerd/containerd/platforms"
 	"io"
 	"io/ioutil"
 	"os"
@@ -44,6 +45,7 @@ func (cmd *buildCommand) Register(fs *flag.FlagSet) {
 	fs.Var(&cmd.tags, "tag", "Name and optionally a tag in the 'name:tag' format")
 	fs.Var(&cmd.tags, "t", "Name and optionally a tag in the 'name:tag' format")
 	fs.StringVar(&cmd.target, "target", "", "Set the target build stage to build")
+	fs.Var(&cmd.platforms, "platform", "Set platforms for which the image should be built")
 	fs.Var(&cmd.buildArgs, "build-arg", "Set build-time variables")
 	fs.Var(&cmd.labels, "label", "Set metadata for an image")
 	fs.BoolVar(&cmd.noConsole, "no-console", false, "Use non-console progress UI")
@@ -56,6 +58,7 @@ type buildCommand struct {
 	labels         stringSlice
 	target         string
 	tags           stringSlice
+	platforms      stringSlice
 
 	contextDir string
 	noConsole  bool
@@ -123,6 +126,11 @@ func (cmd *buildCommand) Run(ctx context.Context, args []string) (err error) {
 		}
 	}
 
+	if len(cmd.platforms) < 1 {
+		cmd.platforms = []string{platforms.DefaultString()}
+	}
+	platforms := strings.Join(cmd.platforms, ",")
+
 	// Create the client.
 	c, err := client.New(stateDir, backend, cmd.getLocalDirs())
 	if err != nil {
@@ -135,6 +143,7 @@ func (cmd *buildCommand) Run(ctx context.Context, args []string) (err error) {
 		// We use the base for filename here because we already set up the local dirs which sets the path in createController.
 		"filename": filepath.Base(cmd.dockerfilePath),
 		"target":   cmd.target,
+		"platform": platforms,
 	}
 	if cmd.noCache {
 		frontendAttrs["no-cache"] = ""

build_test.go

@@ -143,3 +143,16 @@ func TestBuildMultipleTags(t *testing.T) {
 		t.FailNow()
 	}
 }
+
+func TestBuildMultiplePlatforms(t *testing.T) {
+	args := []string{"build", "--platform", "amd64", "--platform", "linux/arm64,linux/arm/v7", "-t", "testbuildplatforms", "-"}
+
+	_, err := doRun(args, withDockerfile(`
+  FROM alpine
+  `))
+
+	if err != nil {
+		t.Logf("img %v failed unexpectedly: %v", args, err)
+		t.FailNow()
+	}
+}

client/workeropt.go

@@ -3,6 +3,7 @@ package client
 import (
 	"context"
 	"fmt"
+	"github.com/moby/buildkit/util/binfmt_misc"
 	"os/exec"
 	"path/filepath"
 	"syscall"
@@ -118,6 +119,15 @@ func (c *Client) createWorkerOpt(withExecutor bool) (opt base.WorkerOpt, err err
 
 	xlabels := base.Labels("oci", c.backend)
 
+	var supportedPlatforms []specs.Platform
+	for _, p := range binfmt_misc.SupportedPlatforms() {
+		parsed, err := platforms.Parse(p)
+		if err != nil {
+			return opt, err
+		}
+		supportedPlatforms = append(supportedPlatforms, platforms.Normalize(parsed))
+	}
+
 	opt = base.WorkerOpt{
 		ID:                 id,
 		Labels:             xlabels,
@@ -128,7 +138,7 @@ func (c *Client) createWorkerOpt(withExecutor bool) (opt base.WorkerOpt, err err
 		Applier:            apply.NewFileSystemApplier(contentStore),
 		Differ:             walking.NewWalkingDiff(contentStore),
 		ImageStore:         imageStore,
-		Platforms:          []specs.Platform{platforms.Normalize(platforms.DefaultSpec())},
+		Platforms:          supportedPlatforms,
 		ResolveOptionsFunc: resolver.NewResolveOptionsFunc(nil),
 	}