Disabling a node in YAML should be explicit

[silvin-lubecki]

Description

A YAML node can be disabled using a "enabled" key with a value:

version: "3.4"
services:
  monitor:
    enabled: "! ${myapp.debug}"

But the code handling this feature says that no value means disabling the node too:

version: "3.4"
services:
  monitor:
    enabled: # monitor is disabled

I suggest we should remove this implicit disable with no value, and error out, as I think that enabling should be explicit too.

⚠️ We should also add unit tests on this part!!

[chris-crone]

@silvin-lubecki I see #244 was merged. Did it take this into account?

[ijc]

The code is now at https://github.com/docker/app/blob/master/render/render.go#L135...L157 (and the key is x-enabled). By my reading it still has the same behaviour which @silvin-lubecki proposed changing.

[shreyasbapat]

Is it available for new contributors?