controller: refactor build inputs to external struct

This patch continues the move to attempt to merge the build.Options
struct into the controllerapi.Options message.

To do this, we extract all the input parameters into a dedicated message
(adding the missing ones, except for the InStream parameter which will
require some additional fiddling around). We also rework the
NamedContexts to allow containing States (by transmitting them as
DefinitionOps), and adding a Linked field to the common options.

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

util/buildflags/context.go

@@ -3,15 +3,16 @@ package buildflags
 import (
 	"strings"
 
+	controllerapi "github.com/docker/buildx/controller/pb"
 	"github.com/docker/distribution/reference"
 	"github.com/pkg/errors"
 )
 
-func ParseContextNames(values []string) (map[string]string, error) {
+func ParseContextNames(values []string) (map[string]*controllerapi.NamedContext, error) {
 	if len(values) == 0 {
 		return nil, nil
 	}
-	result := make(map[string]string, len(values))
+	result := make(map[string]*controllerapi.NamedContext, len(values))
 	for _, value := range values {
 		kv := strings.SplitN(value, "=", 2)
 		if len(kv) != 2 {
@@ -22,7 +23,7 @@ func ParseContextNames(values []string) (map[string]string, error) {
 			return nil, errors.Wrapf(err, "invalid context name %s", kv[0])
 		}
 		name := strings.TrimSuffix(reference.FamiliarString(named), ":latest")
-		result[name] = kv[1]
+		result[name] = &controllerapi.NamedContext{Path: kv[1]}
 	}
 	return result, nil
 }