Support reading from env on bake --set <t>.args

This works just like the `build` command where if you have `--build-arg
FOO`, it will read the variable from env and only set a value if the
variable is defined.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

bake/bake.go

@@ -107,16 +107,17 @@ func mergeConfig(c1, c2 Config) Config {
 
 func (c Config) setOverrides(v []string) error {
 	for _, v := range v {
+
 		parts := strings.SplitN(v, "=", 2)
-		if len(parts) != 2 {
-			return errors.Errorf("invalid override %s, expected target.name=value", v)
-		}
 		keys := strings.SplitN(parts[0], ".", 3)
 		if len(keys) < 2 {
 			return errors.Errorf("invalid override key %s, expected target.name", parts[0])
 		}
 
 		name := keys[0]
+		if len(parts) != 2 && keys[1] != "args" {
+			return errors.Errorf("invalid override %s, expected target.name=value", v)
+		}
 
 		t, ok := c.Target[name]
 		if !ok {
@@ -135,7 +136,14 @@ func (c Config) setOverrides(v []string) error {
 			if t.Args == nil {
 				t.Args = map[string]string{}
 			}
+			if len(parts) < 2 {
+				v, ok := os.LookupEnv(keys[2])
+				if ok {
+					t.Args[keys[2]] = v
+				}
+			} else {
 				t.Args[keys[2]] = parts[1]
+			}
 		case "labels":
 			if len(keys) != 3 {
 				return errors.Errorf("invalid key %s, lanels requires name", parts[0])

bake/bake_test.go

@@ -29,11 +29,50 @@ target "webapp" {
 
 	ctx := context.TODO()
 
+	t.Run("NoOverrides", func(t *testing.T) {
 		m, err := ReadTargets(ctx, []string{fp}, []string{"webapp"}, nil)
 		require.NoError(t, err)
 
 		require.Equal(t, "Dockerfile.webapp", *m["webapp"].Dockerfile)
 		require.Equal(t, ".", *m["webapp"].Context)
+	})
+
+	t.Run("ArgsOverrides", func(t *testing.T) {
+		os.Setenv("VAR_FROMENV"+t.Name(), "fromEnv")
+		defer os.Unsetenv("VAR_FROM_ENV" + t.Name())
+
+		m, err := ReadTargets(ctx, []string{fp}, []string{"webapp"}, []string{
+			"webapp.args.VAR_UNSET",
+			"webapp.args.VAR_EMPTY=",
+			"webapp.args.VAR_SET=bananas",
+			"webapp.args.VAR_FROMENV" + t.Name(),
+		})
+		require.NoError(t, err)
+
+		require.Equal(t, "Dockerfile.webapp", *m["webapp"].Dockerfile)
+		require.Equal(t, ".", *m["webapp"].Context)
+
+		_, isSet := m["webapp"].Args["VAR_UNSET"]
+		require.False(t, isSet, m["webapp"].Args["VAR_UNSET"])
+
+		_, isSet = m["webapp"].Args["VAR_EMPTY"]
+		require.True(t, isSet, m["webapp"].Args["VAR_EMPTY"])
+
+		require.Equal(t, m["webapp"].Args["VAR_SET"], "bananas")
+
+		require.Equal(t, m["webapp"].Args["VAR_FROMENV"+t.Name()], "fromEnv")
+	})
+
+	t.Run("ContextOverride", func(t *testing.T) {
+		_, err := ReadTargets(ctx, []string{fp}, []string{"webapp"}, []string{"webapp.context"})
+		require.NotNil(t, err)
+
+		m, err := ReadTargets(ctx, []string{fp}, []string{"webapp"}, []string{"webapp.context=foo"})
+		require.NoError(t, err)
+
+		require.Equal(t, "foo", *m["webapp"].Context)
+	})
+
 }
 
 func TestReadTargetsCompose(t *testing.T) {