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.

.devcontainer/devcontainer.json

@@ -1,12 +1,12 @@
 {
-    "name": "Ubuntu",
+    "name": "SerenityOS",
     "image": "mcr.microsoft.com/devcontainers/base:jammy",
 
     // Features to add to the dev container. More info: https://containers.dev/implementors/features.
     "features": {
         "ghcr.io/devcontainers/features/github-cli:1": {},
         "ghcr.io/devcontainers-contrib/features/pre-commit:1": {},
-        "./serenity": {
+        "./features/serenity": {
             "llvm_version": 15,
             "enable_ladybird": true,
             "enable_serenity": true

.devcontainer/optimized/.devcontainer.json

@@ -0,0 +1,11 @@
+// The docker image used below was generated from '.devcontainer/devcontainer.json'
+// by the '.github/workflows/dev-container.yml' workflow.
+//
+// By building this dev container image in advance, tools 
+// like GitHub Codespaces (https://containers.dev/supporting) 
+// do not need to install all the prerequsite dependencies from scratch, 
+// getting you into your development environment faster!
+{
+    "name": "SerenityOS (Pre-Built Image)",
+    "image": "ghcr.io/SerenityOS/serenity-devcontainer:base"
+}

.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