Add env. prefix

Author: darnfish

.github/workflows/master.yml

@@ -11,4 +11,4 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v2.0.0
-      - uses: ${{ GITHUB_REPOSITORY }}/.github/actions/docker@master
+      - uses: ${{ env.GITHUB_REPOSITORY }}/.github/actions/docker@master