bake: various fixes for composable attributes

This changes how the composable attributes are implemented and provides
various fixes to the first iteration.

Cache-from and cache-to now no longer print sensitive values that are
automatically added. These automatically added attributes are added when
the protobuf is created rather than at the time of parsing so they will
no longer be printed. If they are part of the original configuration
file, they will still be printed.

Empty strings will now be skipped. This was the original behavior and
composable attributes removed this functionality accidentally. This
functionality is now restored.

This also expands the available syntax that works with each of the
composable attributes. It is now possible to interleave the csv syntax
with the object syntax without any problems. The canonical form is still
the object syntax and variables are resolved according to that syntax.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

util/buildflags/secrets.go

@@ -8,6 +8,39 @@ import (
 	"github.com/tonistiigi/go-csvvalue"
 )
 
+type Secrets []*Secret
+
+func (s Secrets) Merge(other Secrets) Secrets {
+	if other == nil {
+		s.Normalize()
+		return s
+	} else if s == nil {
+		other.Normalize()
+		return other
+	}
+
+	return append(s, other...).Normalize()
+}
+
+func (s Secrets) Normalize() Secrets {
+	if len(s) == 0 {
+		return nil
+	}
+	return removeDupes(s)
+}
+
+func (s Secrets) ToPB() []*controllerapi.Secret {
+	if len(s) == 0 {
+		return nil
+	}
+
+	entries := make([]*controllerapi.Secret, len(s))
+	for i, entry := range s {
+		entries[i] = entry.ToPB()
+	}
+	return entries
+}
+
 type Secret struct {
 	ID       string `json:"id,omitempty"`
 	FilePath string `json:"src,omitempty"`
@@ -85,6 +118,10 @@ func (s *Secret) UnmarshalText(text []byte) error {
 func ParseSecretSpecs(sl []string) ([]*controllerapi.Secret, error) {
 	fs := make([]*controllerapi.Secret, 0, len(sl))
 	for _, v := range sl {
+		if v == "" {
+			continue
+		}
+
 		s, err := parseSecret(v)
 		if err != nil {
 			return nil, err