Make devcontainer base image configurable via BASE_IMAGE build arg

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: chapterjason

.devcontainer/Dockerfile

@@ -1,4 +1,5 @@
-FROM debian:stable-slim
+ARG BASE_IMAGE=ghcr.io/sourecode/devcontainer-base:latest
+FROM ${BASE_IMAGE}
 
 ARG USERNAME
 ARG USER_UID=1000

.devcontainer/devcontainer.json

@@ -3,6 +3,7 @@
   "build": {
     "dockerfile": "Dockerfile",
     "args": {
+      "BASE_IMAGE": "${localEnv:DEVCONTAINER_BASE_IMAGE:ghcr.io/sourecode/devcontainer-base:latest}",
       "USERNAME": "${localEnv:DEVCONTAINER_USERNAME:dev}",
       "USER_UID": "${localEnv:DEVCONTAINER_USER_UID:1000}",
       "USER_GID": "${localEnv:DEVCONTAINER_USER_GID:1000}"