"auth": {
    "type": "BASIC",
    "config": {
      "username": "${APIMAN_AUTH_USERNAME}",
      "password": "${APIMAN_AUTH_PASSWORD}"
    }
  },

The username required for authentication will actually be "${APIMAN_AUTH_USERNAME}", same for password. No substitution is performed.

Env variable substitution seems useful when dealing with passwords. Especially for the purpose of docker image this would allow the ability to change the credentials.