fix(docker): clone platform from git instead of additional_contexts

The additional_contexts approach required cc-platform-core to exist on
the Docker host at a relative path. This fails on servers where the
repo layout differs. Instead, use a multi-stage build: stage 1 clones
cc-platform-core from Gitea (depth 1), stage 2 copies server/ into the
app and rewrites the file: path. Fully self-contained — no host deps.

web/docker-compose.yml

@@ -1,9 +1,6 @@
 services:
   server:
-    build:
-      context: ./server
-      additional_contexts:
-        platform-server: ../../cc-platform-core/server
+    build: ./server
     networks:
       - inventory-network
     volumes: