dockerfile: allow required flag to take value

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

frontend/dockerfile/dockerfile_secrets_test.go

@@ -29,7 +29,7 @@ func testSecretFileParams(t *testing.T, sb integration.Sandbox) {
 
 	dockerfile := []byte(`
 FROM busybox
-RUN --mount=type=secret,mode=741,uid=100,gid=102,target=/mysecret [ "$(stat -c "%u %g %f" /mysecret)" = "100 102 81e1" ]
+RUN --mount=type=secret,required=false,mode=741,uid=100,gid=102,target=/mysecret [ "$(stat -c "%u %g %f" /mysecret)" = "100 102 81e1" ]
 `)
 
 	dir, err := tmpdir(

frontend/dockerfile/instructions/commands_runmount.go

@@ -176,6 +176,14 @@ func parseMount(value string) (*Mount, error) {
 			}
 			m.ReadOnly = !rw
 			roAuto = false
+		case "required":
+			if m.Type == "secret" || m.Type == "ssh" {
+				v, err := strconv.ParseBool(value)
+				if err != nil {
+					return nil, errors.Errorf("invalid value for %s: %s", key, value)
+				}
+				m.Required = v
+			}
 		case "id":
 			m.CacheID = value
 		case "sharing":