Use erikmd/docker-coq-action@alpha

following the merge of PR rocq-community/docker-coq-action#5

Author: erikmd

.github/workflows/build-coq-demo.yml

@@ -27,7 +27,7 @@ jobs:
     # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
     - uses: actions/checkout@v2
 
-    - uses: erikmd/docker-coq-action@feature-timegroup
+    - uses: erikmd/docker-coq-action@alpha
       id: docker-coq-action
       with:
         opam_file: 'coq-demo.opam'