Merge pull request #5 from Doist/deorus/gh-workflow-set-output

ci: Remove deprecated set-output usage

.github/workflows/docker.yml

@@ -28,7 +28,7 @@ jobs:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
       - uses: actions/checkout@v2
       - run: |
-          echo ::set-output name=short::$(git rev-parse --short=7 ${{ github.sha }})
+          echo short=$(git rev-parse --short=7 ${{ github.sha }}) >> $GITHUB_OUTPUT
         id: git
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v1