Use erikmd/docker-coq-action@alpha

following the merge of PRs:

- rocq-community/docker-coq-action#8
- rocq-community/docker-coq-action#9

Author: erikmd

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

@@ -4,12 +4,12 @@ name: CI
 
 # Controls when the action will run.
 # cf. https://help.github.com/en/actions/configuring-and-managing-workflows/configuring-a-workflow#filtering-for-specific-branches-tags-and-paths
-# Triggers the workflow on all push events or pull request events for the
+# Triggers the workflow on push events or pull request events for the
 # master branch only:
 on:
   push:
     branches:
-      - '**'
+      - master
   pull_request:
     branches:
       - master
@@ -25,7 +25,7 @@ jobs:
     strategy:
       matrix:
         coq_version: ['8.11', 'dev']
-        ocaml_version: ['4.07-flambda', '4.09-flambda']
+        ocaml_version: ['4.07-flambda']
       # at most 20 concurrent jobs per free account
       # cf. https://help.github.com/en/actions/reference/workflow-syntax-for-github-actions#usage-limits
       max-parallel: 4
@@ -35,7 +35,7 @@ jobs:
     # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
     - uses: actions/checkout@v2
 
-    - uses: erikmd/docker-coq-action@simplify-entrypoint
+    - uses: erikmd/docker-coq-action@alpha
       id: docker-coq-action
       with:
         opam_file: 'coq-demo.opam'
@@ -50,8 +50,6 @@ jobs:
             opam update -y
           endGroup
           startGroup Build
-            variable=OK
-            echo "$variable"
             opam install -y -v -j 2 $PACKAGE
             opam list
           endGroup