Yes cucumbers are perfectly safe for dogs to eat.
I share some cucumbers with my dog whenever I eat cucumbers myself and he loves the cucumbers.
Cucumbers are very healthy for both humans and dogs as well as cats.
When you feed the dog cucumbers you should cut the cucumbers up in slices or bite sized pieces so that your dog can more easily chew and digest the cucumbers.
But adding some cucumbers to your dogs diet or feeding the dog cucumbers as a snack is perfectly fine although a whole cucumber may be too much for the dog to eat at once.
But feeding the dog cucumbers in moderation and when sliced up is very good for your dog.