build: basis of build command

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

build/platform.go

@@ -0,0 +1,32 @@
+package build
+
+import (
+	"strings"
+
+	"github.com/containerd/containerd/platforms"
+	specs "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+func ParsePlatformSpecs(platformsStr []string) ([]specs.Platform, error) {
+	if len(platformsStr) == 0 {
+		return nil, nil
+	}
+	out := make([]specs.Platform, 0, len(platformsStr))
+	for _, s := range platformsStr {
+		parts := strings.Split(s, ",")
+		if len(parts) > 1 {
+			p, err := ParsePlatformSpecs(parts)
+			if err != nil {
+				return nil, err
+			}
+			out = append(out, p...)
+			continue
+		}
+		p, err := platforms.Parse(s)
+		if err != nil {
+			return nil, err
+		}
+		out = append(out, platforms.Normalize(p))
+	}
+	return out, nil
+}