feat: consolidate package Docker images into shared package-ci stage

Replace per-package Dockerfiles with a shared multi-stage build in the
root Dockerfile. Packages now reference the shared image and mount
monorepo dependencies as volumes for local development.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: zajca

Dockerfile

@@ -1,14 +1,18 @@
 services:
-  dev: &dev
+  dev:
     build:
-      context: .
+      context: ../..
+      dockerfile: Dockerfile
+      target: package-ci
     image: keboola/storage-api-client
     volumes:
       - ./:/code
 
   dev-xdebug:
-    <<: *dev
     build:
-      context: docker/xdebug
+      context: ../..
+      dockerfile: Dockerfile
+      target: package-ci-xdebug
     image: keboola/storage-api-client-xdebug
-    command: tail -f /dev/null
+    volumes:
+      - ./:/code