compose: propagate BASE_IMAGE build-arg via env; verified default and mirror BuildKit builds

docker-compose.dev.yml

@@ -5,6 +5,8 @@ services:
     build: 
       context: .
       dockerfile: Dockerfile
+      args:
+        BASE_IMAGE: ${BASE_IMAGE:-python:3.12-slim}
     container_name: delphi-database-dev
     ports:
       - "${EXTERNAL_PORT:-6920}:8000"

docker-compose.yml

@@ -1,6 +1,9 @@
 services:
   delphi-db:
-    build: .
+    build:
+      context: .
+      args:
+        BASE_IMAGE: ${BASE_IMAGE:-python:3.12-slim}
     container_name: delphi-database
     ports:
       - "${EXTERNAL_PORT:-6920}:8000"