fix: align container image naming convention

Use ghcr.io/host-uk/core-images:{tag} format where tag is the
image variant (developer, server-php) rather than path-based naming.

See: https://github.com/host-uk/core-images/issues/1

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.devcontainer/devcontainer.json

@@ -1,6 +1,6 @@
 {
   "name": "Core Developer",
-  "image": "ghcr.io/host-uk/core-images/developer:latest",
+  "image": "ghcr.io/host-uk/core-images:developer",
   
   "workspaceMount": "source=${localWorkspaceFolder}/packages,target=/workspace,type=bind",
   "workspaceFolder": "/workspace",

docker-compose.yml

@@ -3,7 +3,7 @@
 
 services:
   dev:
-    image: ghcr.io/host-uk/core-images/developer:latest
+    image: ghcr.io/host-uk/core-images:developer
     container_name: core-dev
     hostname: core-dev
     stdin_open: true