fix: switch Docker registry from Docker Hub to GHCR to fix push authorization failure

The Docker Hub push failed with 'insufficient_scope: authorization failed'.
Switch to GitHub Container Registry (GHCR) which uses the built-in
GITHUB_TOKEN and requires no external secrets configuration.

- Use ghcr.io registry with GITHUB_TOKEN authentication
- Add packages:write permission to workflow
- Update compose.yaml image reference to GHCR

Agent-Logs-Url: https://github.com/Baker-link-Lab/bakerlink_tutorial_template/sessions/b218a431-c3b4-484b-a1f5-9de7fc7ec76b

Co-authored-by: Baker-Tanaka <42606588+Baker-Tanaka@users.noreply.github.com>

Author: Copilot

.docker/compose.yaml

@@ -1,6 +1,6 @@
 services:
   {{project-name}}:
-    image: "bakerlinklab/bakerlink-env:latest"
+    image: "ghcr.io/baker-link-lab/bakerlink-env:latest"
     container_name: "{{project-name}}-container"
     volumes:
       - "../:/{{project-name}}"

.github/workflows/docker-publish.yml

@@ -8,26 +8,28 @@ on:
   workflow_dispatch:
 
 env:
-  DOCKER_IMAGE: bakerlinklab/bakerlink-env
+  REGISTRY: ghcr.io
+  DOCKER_IMAGE: ghcr.io/baker-link-lab/bakerlink-env
 
 jobs:
   build-and-push:
     runs-on: ubuntu-latest
     permissions:
       contents: read
+      packages: write
     steps:
       - name: Checkout
         uses: actions/checkout@v4
 
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
-      - name: Login to Docker Hub
-        # Requires repository secrets: DOCKERHUB_USERNAME and DOCKERHUB_TOKEN
+      - name: Login to GitHub Container Registry
         uses: docker/login-action@v3
         with:
-          username: ${{ secrets.DOCKERHUB_USERNAME }}
-          password: ${{ secrets.DOCKERHUB_TOKEN }}
+          registry: ${{ env.REGISTRY }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Build and push
         uses: docker/build-push-action@v6