Meta: Prebuild repo dev container

This commit adds a GitHub Workflow to prebuild the dev container file
present at .devcontainer/devcontainer.json. This prebuilt image is
pushed to GitHub Container Registry (ghcr).

An additional devcontainer.json is added consuming that image.
This dev container can be selected in editors that process dev
containers (ie GitHub Codespaces), to speed up time to editor.

.github/workflows/dev-container.yml

@@ -0,0 +1,39 @@
+# This workflow builds a docker image with the Dev Container CLI (https://github.com/devcontainers/cli)
+# 
+name: 'Build Dev Container Image' 
+on:
+  workflow_dispatch:
+  push:
+    paths:
+      - '.devcontainer/**'
+  schedule:
+    # https://crontab.guru/#0_0_*_*_1
+    - cron:  '0 0 * * 1'
+  
+
+permissions:
+  contents: read
+  # Push images to GHCR.
+  packages: write
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v2 
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build Base Dev Container Image
+        uses: devcontainers/ci@v0.3
+        with:
+          imageName: ghcr.io/${{ github.repository_owner }}/serenity-devcontainer
+          imageTag: base,latest
+          push: always