devcontainer: Build codespaces image and push them in new action

.devcontainer/compose.yaml

@@ -1,6 +1,7 @@
 services:
   slackbot:
-    build: slackbot
+    # build: slackbot
+    image: ghcr.io/tsg-ut/slackbot-codespaces-image-slackbot:master
     volumes:
       - ..:/code
       - /code/node_modules

.github/workflows/codespaces-image.yml

@@ -0,0 +1,46 @@
+name: Create and publish Codespaces Docker images
+
+on:
+  push:
+    branches:
+      - master
+      - codespaces-image-build
+
+env:
+  BRANCH_NAME: ${{ github.head_ref || github.ref_name }} 
+
+jobs:
+  build-and-push-image:
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: read
+      packages: write
+      attestations: write
+      id-token: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Log in to the Container registry
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push Docker image (slackbot)
+        id: push_slackbot
+        uses: docker/build-push-action@v6
+        with:
+          context: .devcontainer/slackbot
+          push: true
+          tags: ghcr.io/tsg-ut/slackbot-codespaces-image-slackbot:master
+
+      - name: Generate artifact attestation (slackbot)
+        uses: actions/attest-build-provenance@v1
+        with:
+          subject-name: ghcr.io/tsg-ut/slackbot-codespaces-image-slackbot:master
+          subject-digest: ${{ steps.push_slackbot.outputs.digest }}
+          push-to-registry: true