This is a followup for the discussion in #71

Lets imagine
BUILD:

k8s_object(
    name = "deployment",
    template = ":deployment.yaml",
    images = {
      "web-frontend": "//src/docker/web_frontend:web_frontend",
    },
  )

deployment.yaml

apiVersion: extensions/v1beta1
kind: Deployment
spec:
  replicas: 1
  template:
    spec:
      containers:
      - name: web-frontend
        image: web-frontend 

Since the rules are using a simple expand_template action and the substitution keys are simple words, this will replace web-frontend in both name and image.

A naive approach would be to make the key in the substitution something like "image: " + spec so that it only applies to image tags. But this would not be robust for whilespaces and formatting variants (template can be yaml and json).

A better approach would be to mark the substitutions (this is what we do). That is in the file write e.g. "${web-frontend}" you wrap the substitutions keys with '${' + '}', but this won't be backwards compatible. I think this would be a much better approach though. Maybe we can add a boolean flag to enable substitution_guards. Or if people write the key in images as '${web-frondend}' you remove the guards to get the actual image name. Then we don't need the flag.