Add unified CI workflow

This commit adds a new unified CI workflow for building and publishing
the Zephyr SDK build Docker image.

The new unified CI workflow comes with the following changes:

1. Use single workflow for building and publishing Docker image

2. Use the official Docker actions for building and publishing Docker
   image

3. Upload Docker image to both DockerHub and GitHub Container Registry
   (GHCR) such that they are always available from both sources.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

.github/workflows/ci.yml

@@ -0,0 +1,64 @@
+name: CI
+
+on:
+  push:
+    branches: [ master ]
+    tags: [ 'v*' ]
+  pull_request:
+    branches: [ master ]
+
+permissions:
+  packages: write
+
+concurrency:
+  group: ${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    name: Build
+    runs-on: ubuntu-20.04
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+
+    - name: Generate metadata
+      id: meta
+      uses: docker/metadata-action@v3
+      with:
+        images: |
+          docker.io/zephyrprojectrtos/sdk-build
+          ghcr.io/zephyrproject-rtos/sdk-build
+
+    - name: Set up QEMU
+      uses: docker/setup-qemu-action@v1
+
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v1
+
+    - name: Login to DockerHub
+      if: ${{ github.event_name != 'pull_request' }}
+      uses: docker/login-action@v1
+      with:
+        registry: docker.io
+        username: ${{ secrets.DOCKERHUB_USERNAME }}
+        password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+    - name: Login to GitHub Container Registry
+      if: ${{ github.event_name != 'pull_request' }}
+      uses: docker/login-action@v1
+      with:
+        registry: ghcr.io
+        username: ${{ github.repository_owner }}
+        password: ${{ secrets.GITHUB_TOKEN }}
+
+    - name: Build and push Docker image
+      uses: docker/build-push-action@v2
+      with:
+        context: .
+        file: Dockerfile
+        platforms: linux/amd64,linux/arm64
+        push: ${{ github.event_name != 'pull_request' }}
+        tags: ${{ steps.meta.outputs.tags }}
+        labels: ${{ steps.meta.outputs.labels }}