Test runner config values not properly detected

The test runner does not properly identify config values that are used in string interpolation. For example, in the following compose file:

services:
  hasura:
    restart: unless-stopped
    build:
      dockerfile: Dockerfile
      context: ./hasura
    # If you want to add your own domain name in Defang BYOC, uncomment the following line and replace the domain name with your own
    # domainname: hasura.mycoolapp.com
    ports:
      - target: 8080
        published: 8080
        mode: ingress
    depends_on:
      database:
        condition: service_started
    environment:
      - HASURA_GRAPHQL_ADMIN_SECRET
      - HASURA_GRAPHQL_DATABASE_URL=postgres://postgres:${POSTGRES_PASSWORD}@database:5432/postgres?sslmode=${SSL_MODE}
      - HASURA_GRAPHQL_ENABLE_CONSOLE=true
      - HASURA_GRAPHQL_UNAUTHORIZED_ROLE=public
      - HASURA_GRAPHQL_EXPERIMENTAL_FEATURES=naming_convention
      - HASURA_GRAPHQL_DEFAULT_NAMING_CONVENTION=graphql-default
      - HASURA_GRAPHQL_MIGRATIONS_DIR=/hasura/migrations
      - HASURA_GRAPHQL_METADATA_DIR=/hasura/metadata
  
  database:
    image: postgres:16
    x-defang-postgres: true
    ports:
      - target: 5432
        mode: host
    environment:
      - POSTGRES_PASSWORD

It does not detect SSL_MODE so tests always fail. The following is the current workaround:

      - SSL_MODE

That allows it to be detected and thus load the correct secret from the test environment.

Reproduce the issue with the provided compose configuration, focusing on the interpolated POSTGRES_PASSWORD and SSL_MODE values. Trace how the test runner discovers environment references and verify that SSL_MODE is detected without adding the standalone workaround entry. Done means the affected tests load SSL_MODE from the test environment and pass.