container: make container.NewResolver() return a pointer

Most go packages return pointers to structs when using `NewFoo()`.
But `NewResolver()` does not and AFAICT there is no deep reason
for this. I would prefer to return a pointer here, I got tripped
up by this in
osbuild/bootc-image-builder#139 (comment)

pkg/container/resolver.go

@@ -31,8 +31,8 @@ type SourceSpec struct {
 	StoragePath         *string
 }
 
-func NewResolver(arch string) Resolver {
-	return Resolver{
+func NewResolver(arch string) *Resolver {
+	return &Resolver{
 		ctx:   context.Background(),
 		queue: make(chan resolveResult, 2),
 		Arch:  arch,