update ci script (#7)

.github/dependabot.yml

@@ -0,0 +1,7 @@
+version: 2
+
+updates:
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly

.github/workflows/main.yml

@@ -1,13 +1,10 @@
-# following https://github.com/coq-community/huffman/blob/master/.github/workflows/docker-action.yml
-# See also:
-# https://github.com/coq-community/docker-coq-action#readme
-# https://github.com/erikmd/docker-coq-github-action-demo
+# following https://github.com/coq-community/docker-coq-action
 
 name: Docker CI
 
 on:
   pull_request:
-    types: [opened, synchronize, edited, reopened]
+    types: [opened, synchronize, reopened]
   push:
   workflow_dispatch:
 
@@ -17,14 +14,14 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        image:
-          - 'coqorg/coq:dev'
-          - 'coqorg/coq:8.20'
-          - 'coqorg/coq:8.19'
+        coq_version:
+          - dev
+          - '8.20'
+          - '8.19'
       fail-fast: false
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: coq-community/docker-coq-action@v1
         with:
           opam_file: 'coq-hol-light.opam'
-          custom_image: ${{ matrix.image }}
+          coq_version: ${{ matrix.coq_version }}