bake: fix global target access when using a matrix

Previously, we would fail while trying to use the global "target" field
when using a matrix. The contents of the matrix really don't matter for
this.

What was happening was that we would copy the "target" property into the
child evaluation context, so that when it was updated on the parent, it
wouldn't propagate to the child. The correct behavior here is to avoid
copying variables from the target evaluation context if it is the
root.

Signed-off-by: Justin Chadwell <me@jedevc.com>

bake/hcl_test.go

@@ -1113,6 +1113,27 @@ func TestHCLMatrixBadTypes(t *testing.T) {
 	require.Error(t, err)
 }
 
+func TestHCLMatrixWithGlobalTarget(t *testing.T) {
+	dt := []byte(`
+		target "x" {
+			tags = ["a", "b"]
+		}
+		
+		target "default" {
+			tags = target.x.tags
+			matrix = {
+				dummy = [""]
+			}
+		}
+	`)
+	c, err := ParseFile(dt, "docker-bake.hcl")
+	require.NoError(t, err)
+	require.Equal(t, 2, len(c.Targets))
+	require.Equal(t, "x", c.Targets[0].Name)
+	require.Equal(t, "default", c.Targets[1].Name)
+	require.Equal(t, []string{"a", "b"}, c.Targets[1].Tags)
+}
+
 func TestJSONAttributes(t *testing.T) {
 	dt := []byte(`{"FOO": "abc", "variable": {"BAR": {"default": "def"}}, "target": { "app": { "args": {"v1": "pre-${FOO}-${BAR}"}} } }`)