Build arguments

Docker build arguments & static service environment variables
Build arguments are key:value pairs that will be used to initialize a build with either docker-compose or static service environment variables. A static service is any service that doesn't require a container. Visit our guide to static service deployment for more details.
Build arguments can be global and used for all your applications, or specific to a single application.
Global build arguments can be kept in your Account Settings under the Builds tab. Secret environment variables that are only relevant to a single application can be added to App Settings in the "Advanced Settings" section so that they aren't exposed to other containers.

Add account-level build arguments

Navigate to Account Settings screen by clicking the sliders icon in the upper right. Select the Builds tab.
Build Arguments Setting
An example use-case for global build arguments is an API key for a static build.
Build Arguments Example
You can also add secrets referenced from SSM or Secrets Manager in the following link Referencing Cloud Secrets

Add application-level build arguments

Navigate to App Settings. Select Advanced Settings. Toggle open the "App-Level Build Args" section.
Navigating to App-Level Build Args under Application Advanced Settings