build: print error if BuildKit/non-BuildKit-specific flags are used

With this patch, the `--progress`, `--secret`, `--ssh`, and `--output` flags
trigger an error when trying to use without BuildKit enabled;

    DOCKER_BUILDKIT=0 docker build --progress=plain .
    --progress is only supported with BuildKit enabled. Enable BuildKit with DOCKER_BUILDKIT=1

    DOCKER_BUILDKIT=0 docker build --output=foo .
    --output is only supported with BuildKit enabled. Enable BuildKit with DOCKER_BUILDKIT=1

Likewise, options that are not supported yet by BuildKit, now trigger an error:

    DOCKER_BUILDKIT=1 docker build --memory=500M .
    --memory is not supported with BuildKit enabled. Disable BuildKit with DOCKER_BUILDKIT=0

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

cmd/docker/docker.go

@@ -420,7 +420,16 @@ func areFlagsSupported(cmd *cobra.Command, details versionDetails) error {
 		if _, ok := f.Annotations["experimentalCLI"]; ok && !details.ClientInfo().HasExperimental {
 			errs = append(errs, fmt.Sprintf(`"--%s" is only supported on a Docker cli with experimental cli features enabled`, f.Name))
 		}
-		// buildkit-specific flags are noop when buildkit is not enabled, so we do not add an error in that case
+		if _, ok := f.Annotations["buildkit"]; ok {
+			if v, _ := command.BuildKitEnabled(details.ServerInfo()); !v {
+				errs = append(errs, fmt.Sprintf(`"--%s" is only supported with BuildKit enabled. Enable BuildKit with DOCKER_BUILDKIT=1`, f.Name))
+			}
+		}
+		if _, ok := f.Annotations["no-buildkit"]; ok {
+			if v, _ := command.BuildKitEnabled(details.ServerInfo()); v {
+				errs = append(errs, fmt.Sprintf(`"--%s" is not supported with BuildKit enabled. Disable BuildKit with DOCKER_BUILDKIT=0`, f.Name))
+			}
+		}
 	})
 	if len(errs) > 0 {
 		return errors.New(strings.Join(errs, "\n"))