How to set environment variables using cloudbuild.yaml for react project

Question

The build is working but environment variables are not assigned during the build, How can I access environment variables?

My cloudbuild.yaml file(Substitution variables are added in the trigger configuration)

steps:
  # Step 1: Build and submit to Google Container Registry
  - name: "gcr.io/cloud-builders/gcloud"
    args:
      [
        "builds",
        "submit",
        "--tag",
        "gcr.io/$PROJECT_ID/image-name:version-name"
      ]
    env:
      - 'REACT_APP_BACKEND_BASE_URL=/'
      - 'REACT_APP_FIREBASE_API_KEY=${_REACT_APP_FIREBASE_API_KEY}'
      - 'REACT_APP_FIREBASE_AUTH_DOMAIN=${_REACT_APP_FIREBASE_AUTH_DOMAIN}'
      - 'REACT_APP_FIREBASE_DATABASE_URL=${_REACT_APP_FIREBASE_DATABASE_URL}'
      - 'REACT_APP_FIREBASE_PROJECT_ID=${_REACT_APP_FIREBASE_PROJECT_ID}'
      - 'REACT_APP_FIREBASE_STORAGE_BUCKET=${_REACT_APP_FIREBASE_STORAGE_BUCKET}'
      - 'REACT_APP_FIREBASE_MESSAGING_SENDERID=${_REACT_APP_FIREBASE_MESSAGING_SENDERID}'
options:
  logging: CLOUD_LOGGING_ONLY