chore: enable weekly build of flutter Dev Container

This commit configures a scheduled GitHub Workflow to periodically build
and push the Dev Container image into the GitHub Container Registry.
This allows to use the prebuilt image directly, thereby dramatically
reducing the time to start and open the project in the Dev Container.

.devcontainer/devcontainer.json

@@ -1,7 +1,9 @@
 {
-  "build": {
-    "dockerfile": "Dockerfile"
-  },
+  "image": "ghcr.io/fossasia/flutter-devcontainer:latest",
+  // Or build locally
+  // "build": {
+  //   "dockerfile": "Dockerfile"
+  // },
   "runArgs": [
     "--add-host",
     "host.docker.internal:host-gateway",

.github/workflows/devcontainer.yml

@@ -0,0 +1,29 @@
+name: Build Dev Container
+
+on:
+  schedule:
+    - cron: '0 0 * * 0'
+  workflow_dispatch:
+
+jobs:
+  devcontainer:
+    runs-on: ubuntu-latest
+    permissions:
+      packages: write
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Pre-build dev container image
+        uses: devcontainers/[email protected]
+        with:
+          imageName: ghcr.io/${{ github.repository_owner }}/flutter-devcontainer
+          imageTag: latest
+          cacheFrom: ghcr.io/${{ github.repository_owner }}/flutter-devcontainer:latest
+          push: always