controller: strongly type the controller api

Strongly typing the API allows us to perform all command line parsing
fully on the client-side, where we have access to the client local
directory and all the client environment variables, which may not be
available on the remote server.

Additionally, the controller api starts to look a lot like
build.Options, so at some point in the future there may be an
oppportunity to merge the two, which would allow both build and bake to
execute through the controller, instead of needing to maintain multiple
code paths.

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

util/buildflags/context.go

@@ -0,0 +1,28 @@
+package buildflags
+
+import (
+	"strings"
+
+	"github.com/docker/distribution/reference"
+	"github.com/pkg/errors"
+)
+
+func ParseContextNames(values []string) (map[string]string, error) {
+	if len(values) == 0 {
+		return nil, nil
+	}
+	result := make(map[string]string, len(values))
+	for _, value := range values {
+		kv := strings.SplitN(value, "=", 2)
+		if len(kv) != 2 {
+			return nil, errors.Errorf("invalid context value: %s, expected key=value", value)
+		}
+		named, err := reference.ParseNormalizedNamed(kv[0])
+		if err != nil {
+			return nil, errors.Wrapf(err, "invalid context name %s", kv[0])
+		}
+		name := strings.TrimSuffix(reference.FamiliarString(named), ":latest")
+		result[name] = kv[1]
+	}
+	return result, nil
+}