dockerfile: handle args with unset values

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

frontend/dockerfile/dockerfile2llb/convert.go

@@ -623,8 +623,10 @@ func dispatchRun(d *dispatchState, c *instructions.RunCommand, proxy *llb.ProxyE
 	env := d.state.Env()
 	opt := []llb.RunOption{llb.Args(args)}
 	for _, arg := range d.buildArgs {
-		env = append(env, fmt.Sprintf("%s=%s", arg.Key, arg.ValueString()))
-		opt = append(opt, llb.AddEnv(arg.Key, arg.ValueString()))
+		if arg.Value != nil {
+			env = append(env, fmt.Sprintf("%s=%s", arg.Key, arg.ValueString()))
+			opt = append(opt, llb.AddEnv(arg.Key, arg.ValueString()))
+		}
 	}
 	opt = append(opt, dfCmd(c))
 	if d.ignoreCache {
@@ -1066,7 +1068,9 @@ func toEnvMap(args []instructions.KeyValuePairOptional, env []string) map[string
 		if _, ok := m[arg.Key]; ok {
 			continue
 		}
-		m[arg.Key] = arg.ValueString()
+		if arg.Value != nil {
+			m[arg.Key] = arg.ValueString()
+		}
 	}
 	return m
 }

frontend/dockerfile/dockerfile2llb/convert_test.go

@@ -116,7 +116,7 @@ func TestToEnvList(t *testing.T) {
 	args = []instructions.KeyValuePairOptional{{Key: "key2", Value: nil}}
 	env = []string{"key1=val1"}
 	resutl = toEnvMap(args, env)
-	assert.Equal(t, map[string]string{"key1": "val1", "key2": ""}, resutl)
+	assert.Equal(t, map[string]string{"key1": "val1"}, resutl)
 
 	// args has duplicated key with env
 	v = "val2"

frontend/dockerfile/dockerfile_test.go

@@ -86,6 +86,7 @@ var allTests = []integration.Test{
 	testCopyWildcardCache,
 	testDockerignoreOverride,
 	testTarExporter,
+	testDefaultEnvWithArgs,
 }
 
 var fileOpTests = []integration.Test{
@@ -146,6 +147,58 @@ func TestIntegration(t *testing.T) {
 	}))...)
 }
 
+func testDefaultEnvWithArgs(t *testing.T, sb integration.Sandbox) {
+	f := getFrontend(t, sb)
+
+	dockerfile := []byte(`
+FROM busybox AS build
+ARG my_arg
+ENV my_arg "${my_arg:-def_val}"
+COPY myscript.sh myscript.sh 
+RUN ./myscript.sh
+FROM scratch
+COPY --from=build /out /out
+`)
+
+	script := []byte(`
+#!/usr/bin/env sh
+echo -n $my_arg > /out
+`)
+
+	dir, err := tmpdir(
+		fstest.CreateFile("Dockerfile", dockerfile, 0600),
+		fstest.CreateFile("myscript.sh", script, 0700),
+	)
+	require.NoError(t, err)
+	defer os.RemoveAll(dir)
+
+	c, err := client.New(context.TODO(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	destDir, err := ioutil.TempDir("", "buildkit")
+	require.NoError(t, err)
+	defer os.RemoveAll(destDir)
+
+	_, err = f.Solve(context.TODO(), c, client.SolveOpt{
+		Exports: []client.ExportEntry{
+			{
+				Type:      client.ExporterLocal,
+				OutputDir: destDir,
+			},
+		},
+		LocalDirs: map[string]string{
+			builder.DefaultLocalNameDockerfile: dir,
+			builder.DefaultLocalNameContext:    dir,
+		},
+	}, nil)
+	require.NoError(t, err)
+
+	dt, err := ioutil.ReadFile(filepath.Join(destDir, "out"))
+	require.NoError(t, err)
+	require.Equal(t, "def_val", string(dt))
+}
+
 func testDockerignoreOverride(t *testing.T, sb integration.Sandbox) {
 	f := getFrontend(t, sb)
 	dockerfile := []byte(`