message: try to explain the way to use private registries a bit better, there could be some confusion around how to configure the password and how to actually consume a private image
diff: @@ -74,7 +74,7 @@ Docker Registry Environment Variables are only available during a Build and are
  This defines a project wide docker registry variable (available in all environments) for the project with id `463`:
  ```
  mutation addDockerRegistryEnv {
  - addEnvVariable(input:{type:PROJECT, typeId:463, scope:DOCKER, name:"MYVARIABLENAME", value:"MyVariableValue"}) {
  + addEnvVariable(input:{type:PROJECT, typeId:463, scope:DOCKER, name:"MY_OWN_REGISTRY_PASSWORD", value:"MySecretPassword"}) {
  id
  }
  }
  @@ -83,7 +83,7 @@ mutation addDockerRegistryEnv {
  This defines a environment id `546` specific docker registry variable (available only in that specific environment):
  ```
  mutation addDockerRegistryEnv {
  - addEnvVariable(input:{type:ENVIRONMENT, typeId:546, scope:DOCKER, name:"MYVARIABLENAME", value:"MyVariableValue"}) {
  + addEnvVariable(input:{type:ENVIRONMENT, typeId:546, scope:DOCKER, name:"MY_OWN_REGISTRY_PASSWORD", value:"MySecretPassword"}) {
  id
  }
  }
  