Use erikmd/docker-coq-action@alpha

following the merge of PR rocq-community/docker-coq-action#10

Author: erikmd

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

@@ -71,7 +71,7 @@ jobs:
       max-parallel: 4
     steps:
     - uses: actions/checkout@v2
-    - uses: erikmd/docker-coq-action@custom_image
+    - uses: erikmd/docker-coq-action@alpha
       id: docker-mathcomp-action
       with:
         opam_file: './coq-demo.opam'