build: add --annotation shortcut flag

This extracts the same logic for parsing annotations from the imagetools
create command, and allows the same flags to be attached to the build
command.

These annotations are then merged into all provided exporters.

Signed-off-by: Justin Chadwell <me@jedevc.com>

util/buildflags/export.go

@@ -2,10 +2,14 @@ package buildflags
 
 import (
 	"encoding/csv"
+	"regexp"
 	"strings"
 
+	"github.com/containerd/containerd/platforms"
 	controllerapi "github.com/docker/buildx/controller/pb"
 	"github.com/moby/buildkit/client"
+	"github.com/moby/buildkit/exporter/containerimage/exptypes"
+	ocispecs "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/pkg/errors"
 )
 
@@ -74,3 +78,45 @@ func ParseExports(inp []string) ([]*controllerapi.ExportEntry, error) {
 	}
 	return outs, nil
 }
+
+func ParseAnnotations(inp []string) (map[exptypes.AnnotationKey]string, error) {
+	// TODO: use buildkit's annotation parser once it supports setting custom prefix and ":" separator
+	annotationRegexp := regexp.MustCompile(`^(?:([a-z-]+)(?:\[([A-Za-z0-9_/-]+)\])?:)?(\S+)$`)
+	annotations := make(map[exptypes.AnnotationKey]string)
+	for _, inp := range inp {
+		k, v, ok := strings.Cut(inp, "=")
+		if !ok {
+			return nil, errors.Errorf("invalid annotation %q, expected key=value", inp)
+		}
+
+		groups := annotationRegexp.FindStringSubmatch(k)
+		if groups == nil {
+			return nil, errors.Errorf("invalid annotation format, expected <type>:<key>=<value>, got %q", inp)
+		}
+
+		typ, platform, key := groups[1], groups[2], groups[3]
+		switch typ {
+		case "":
+		case exptypes.AnnotationIndex, exptypes.AnnotationIndexDescriptor, exptypes.AnnotationManifest, exptypes.AnnotationManifestDescriptor:
+		default:
+			return nil, errors.Errorf("unknown annotation type %q", typ)
+		}
+
+		var ociPlatform *ocispecs.Platform
+		if platform != "" {
+			p, err := platforms.Parse(platform)
+			if err != nil {
+				return nil, errors.Wrapf(err, "invalid platform %q", platform)
+			}
+			ociPlatform = &p
+		}
+
+		ak := exptypes.AnnotationKey{
+			Type:     typ,
+			Platform: ociPlatform,
+			Key:      key,
+		}
+		annotations[ak] = v
+	}
+	return annotations, nil
+}