Conditional feature properties and option values in config

[evilhamsterman]

I've run into several cases where it would have been nice to be able to make a property of a feature conditional on an option.

For example there are multiple "Persistence" features. These setup volumes to cache data like authentication tokens for apps like the GitHub, Azure, AWS clis. The logic of setting up persistence can easily be handled in install.sh, however they have to be separate feature from the feature to install said cli tools because the mounts property is either specified as part of the feature or not.

If mounts or individual mounts were able to be made conditional on an option, that functionality could be built into the installer feature, and make it easier to do similar for other apps. Even just allowing accessing option and other values in other properties could be helpful for dynamic options.

Example below would create a dynamic volume depending on whether persistence is enable or not. Or it would bind an local path to an internal path based on an option.

{
  "options": {
    "persistence": {
      "type": "boolean",
      "description": "Should persistence be enable for authentication token",
      "default": true
    },
    "tokenpath": {
      "type": "string",
      "description": "Path to your existing token files",
      "default": ""
    }
  },
  "mounts": [
    {
      "source": "auth-token-volume",
      "target": "/dc/auth-token",
      "type": "volume",
      "enable": "${option:persistence}"
    },
    {
      "source": "${option:tokenpath}",
      "target": "/dc/bound-token",
      "type": "bind"
    },
    {
      "source": "${localEnv:APP_AUTH_PATH}",
      "target": "/dc/env-bound-token",
      "type": "bind"
    }
  ],
  "containerEnv": {
    "APP_AUTH_PATH": "/dc/bound-token"
  }
}