Support nerdctl build custom outputs alias of type local

Signed-off-by: Hanchin Hsieh <[email protected]>

cmd/nerdctl/build.go

@@ -290,6 +290,11 @@ func generateBuildctlArgs(cmd *cobra.Command, buildkitHost string, platform, arg
 			needsLoading = true
 		}
 	} else {
+		if !strings.Contains(output, "type=") {
+			// should accept --output <DIR> as an alias of --output
+			// type=local,dest=<DIR>
+			output = fmt.Sprintf("type=local,dest=%s", output)
+		}
 		if strings.Contains(output, "type=docker") || strings.Contains(output, "type=oci") {
 			needsLoading = true
 		}

cmd/nerdctl/build_test.go

@@ -174,9 +174,11 @@ CMD ["echo", "nerdctl-build-test-dockerfile"]
 
 func TestBuildLocal(t *testing.T) {
 	t.Parallel()
-	testutil.DockerIncompatible(t)
 	testutil.RequiresBuild(t)
 	base := testutil.NewBase(t)
+	if testutil.GetTarget() == testutil.Docker {
+		base.Env = append(base.Env, "DOCKER_BUILDKIT=1")
+	}
 	defer base.Cmd("builder", "prune").Run()
 	const testFileName = "nerdctl-build-test"
 	const testContent = "nerdctl"
@@ -202,6 +204,16 @@ COPY %s /`,
 	data, err := os.ReadFile(testFilePath)
 	assert.NilError(t, err)
 	assert.Equal(t, string(data), testContent)
+
+	aliasOutputDir := t.TempDir()
+	testAliasFilePath := filepath.Join(aliasOutputDir, testFileName)
+	base.Cmd("build", "-o", aliasOutputDir, buildCtx).AssertOK()
+	if _, err := os.Stat(testAliasFilePath); err != nil {
+		t.Fatal(err)
+	}
+	data, err = os.ReadFile(testAliasFilePath)
+	assert.NilError(t, err)
+	assert.Equal(t, string(data), testContent)
 }
 
 func createBuildContext(dockerfile string) (string, error) {