feat: cacheing images

[jakeyheath]

I have repo that currently takes a very long time to build the Docker container due to a large npm install. Whenever I run happy create or happy update, it tries to rebuild the images. Is this by design? Here is the use case where this was frustrating:

1. I create some Terraform
2. I fixed some code
3. I create a stack to test my changes
4. I realize the Terraform is not right, so I make an update to the terraform
5. I run happy update <stack_name>
6. Even though I can use the same image, it rebuilds it