bake: fix panic for unsupported hcl variable type

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

bake/hcl_test.go

@@ -782,3 +782,15 @@ func TestFunctionNoResult(t *testing.T) {
 	_, err := ParseFile(dt, "docker-bake.hcl")
 	require.Error(t, err)
 }
+
+func TestVarUnsupportedType(t *testing.T) {
+	dt := []byte(`
+		variable "FOO" {
+			default = []
+		}
+		target "default" {}`)
+
+	t.Setenv("FOO", "bar")
+	_, err := ParseFile(dt, "docker-bake.hcl")
+	require.Error(t, err)
+}

bake/hclparser/hclparser.go

@@ -241,7 +241,7 @@ func (p *parser) resolveValue(name string) (err error) {
 			return nil
 		} else {
 			// TODO: support lists with csv values
-			return errors.Errorf("unsupported type %s for variable %s", v.Type(), name)
+			return errors.Errorf("unsupported type %s for variable %s", vv.Type().FriendlyName(), name)
 		}
 	}
 	v = &vv