Merge pull request #30 from infosiftr/scratch-platform

Set DOCKER_DEFAULT_PLATFORM when building "FROM scratch" images

architecture/oci-platform.go

@@ -1,5 +1,7 @@
 package architecture
 
+import "path"
+
 // https://github.com/opencontainers/image-spec/blob/v1.0.1/image-index.md#image-index-property-descriptions
 // see "platform" (under "manifests")
 type OCIPlatform struct {
@@ -25,3 +27,12 @@ var SupportedArches = map[string]OCIPlatform{
 
 	"windows-amd64": {OS: "windows", Architecture: "amd64"},
 }
+
+// https://pkg.go.dev/github.com/containerd/containerd/platforms
+func (p OCIPlatform) String() string {
+	return path.Join(
+		p.OS,
+		p.Architecture,
+		p.Variant,
+	)
+}

cmd/bashbrew/cmd-build.go

@@ -48,7 +48,9 @@ func cmdBuild(c *cli.Context) error {
 				return cli.NewMultiError(fmt.Errorf(`failed fetching/scraping FROM for %q (tags %q)`, r.RepoName, entry.TagsString()), err)
 			}
 
+			fromScratch := false
 			for _, from := range froms {
+				fromScratch = fromScratch || from == "scratch"
 				if from != "scratch" && pull != "never" {
 					doPull := false
 					switch pull {
@@ -93,7 +95,14 @@ func cmdBuild(c *cli.Context) error {
 
 					// TODO use "meta.StageNames" to do "docker build --target" so we can tag intermediate stages too for cache (streaming "git archive" directly to "docker build" makes that a little hard to accomplish without re-streaming)
 
-					err = dockerBuild(cacheTag, entry.ArchFile(arch), archive)
+					var extraEnv []string = nil
+					if fromScratch {
+						// https://github.com/docker/cli/blob/v20.10.7/cli/command/image/build.go#L163
+						extraEnv = []string{"DOCKER_DEFAULT_PLATFORM=" + ociArch.String()}
+						// ideally, we would set this via an explicit "--platform" flag on "docker build", but it's not supported without buildkit until 20.10+ and this is a trivial way to get Docker to do the right thing in both cases without explicitly trying to detect whether we're on 20.10+
+					}
+
+					err = dockerBuild(cacheTag, entry.ArchFile(arch), archive, extraEnv)
 					if err != nil {
 						return cli.NewMultiError(fmt.Errorf(`failed building %q (tags %q)`, r.RepoName, entry.TagsString()), err)
 					}

cmd/bashbrew/docker.go

@@ -237,10 +237,16 @@ func (r Repo) dockerBuildUniqueBits(entry *manifest.Manifest2822Entry) ([]string
 	return uniqueBits, nil
 }
 
-func dockerBuild(tag string, file string, context io.Reader) error {
-	args := []string{"build", "-t", tag, "-f", file, "--rm", "--force-rm"}
+func dockerBuild(tag string, file string, context io.Reader, extraEnv []string) error {
+	args := []string{"build", "--tag", tag, "--file", file, "--rm", "--force-rm"}
 	args = append(args, "-")
 	cmd := exec.Command("docker", args...)
+	if extraEnv != nil {
+		cmd.Env = append(os.Environ(), extraEnv...)
+		if debugFlag {
+			fmt.Printf("$ export %q\n", extraEnv)
+		}
+	}
 	cmd.Stdin = context
 	if debugFlag {
 		cmd.Stdout = os.Stdout

cmd/bashbrew/main.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/urfave/cli"
 
+	"github.com/docker-library/bashbrew/architecture"
 	"github.com/docker-library/bashbrew/manifest"
 )
 
@@ -22,6 +23,7 @@ var (
 	defaultCache   string
 
 	arch                 string
+	ociArch              architecture.OCIPlatform
 	namespace            string
 	constraints          []string
 	exclusiveConstraints bool
@@ -166,6 +168,11 @@ func main() {
 			constraints = c.GlobalStringSlice("constraint")
 			exclusiveConstraints = c.GlobalBool("exclusive-constraints")
 
+			var ok bool
+			if ociArch, ok = architecture.SupportedArches[arch]; !ok {
+				return fmt.Errorf("invalid architecture: %q", arch)
+			}
+
 			archNamespaces = map[string]string{}
 			for _, archMapping := range c.GlobalStringSlice("arch-namespace") {
 				splitArchMapping := strings.SplitN(archMapping, "=", 2)