cache fix: stable ordering of maps (Env, Mount, ...)

Maps were causing the same Pipeline to randomly produce slightly different LLB
on each run (because they are represented as an array in LLB, wheras
they're a map in CueLLB).

This forces every Cue field iteration (env, mount, etc) to be
predictable by using stable sorting.

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

dagger/compiler/value.go

@@ -1,6 +1,8 @@
 package compiler
 
 import (
+	"sort"
+
 	"cuelang.org/go/cue"
 	cueformat "cuelang.org/go/cue/format"
 )
@@ -72,9 +74,33 @@ func (v *Value) Kind() cue.Kind {
 	return v.val.Kind()
 }
 
+// Field represents a struct field
+type Field struct {
+	Label string
+	Value *Value
+}
+
 // Proxy function to the underlying cue.Value
-func (v *Value) Fields() (*cue.Iterator, error) {
-	return v.val.Fields()
+// Field ordering is guaranteed to be stable.
+func (v *Value) Fields() ([]Field, error) {
+	it, err := v.val.Fields()
+	if err != nil {
+		return nil, err
+	}
+
+	fields := []Field{}
+	for it.Next() {
+		fields = append(fields, Field{
+			Label: it.Label(),
+			Value: v.Wrap(it.Value()),
+		})
+	}
+
+	sort.SliceStable(fields, func(i, j int) bool {
+		return fields[i].Label < fields[j].Label
+	})
+
+	return fields, nil
 }
 
 // Proxy function to the underlying cue.Value
@@ -121,38 +147,10 @@ func (v *Value) List() ([]*Value, error) {
 	for it.Next() {
 		l = append(l, v.Wrap(it.Value()))
 	}
+
 	return l, nil
 }
 
-// FIXME: deprecate to simplify
-func (v *Value) RangeList(fn func(int, *Value) error) error {
-	it, err := v.val.List()
-	if err != nil {
-		return err
-	}
-	i := 0
-	for it.Next() {
-		if err := fn(i, v.Wrap(it.Value())); err != nil {
-			return err
-		}
-		i++
-	}
-	return nil
-}
-
-func (v *Value) RangeStruct(fn func(string, *Value) error) error {
-	it, err := v.Fields()
-	if err != nil {
-		return err
-	}
-	for it.Next() {
-		if err := fn(it.Label(), v.Wrap(it.Value())); err != nil {
-			return err
-		}
-	}
-	return nil
-}
-
 // FIXME: receive string path?
 func (v *Value) Merge(x interface{}, path ...string) (*Value, error) {
 	if xval, ok := x.(*Value); ok {