diff --git a/.github/workflows/build-coq-demo.yml b/.github/workflows/build-coq-demo.yml
index 284ceb9..d76236e 100644
--- a/.github/workflows/build-coq-demo.yml
+++ b/.github/workflows/build-coq-demo.yml
@@ -23,11 +23,11 @@ jobs:
     strategy:
       matrix:
         coq_version:
-          - '8.11'
+          # - '8.11'
           - dev
         ocaml_version:
           - '4.07-flambda'
-          - '4.09-flambda'
+          # - '4.09-flambda'
       # at most 20 concurrent jobs per free account:
       # https://help.github.com/en/actions/reference/workflow-syntax-for-github-actions#usage-limits
       max-parallel: 4
@@ -49,10 +49,12 @@ jobs:
     strategy:
       matrix:
         image:
-          - mathcomp/mathcomp:1.10.0-coq-8.10
+          # - mathcomp/mathcomp:1.10.0-coq-8.10
           - mathcomp/mathcomp:1.10.0-coq-8.11
-          - mathcomp/mathcomp:1.11.0-coq-dev
-          - mathcomp/mathcomp-dev:coq-dev
+          # - mathcomp/mathcomp:1.11.0-coq-dev  # not available anymore
+          # - mathcomp/mathcomp-dev:coq-dev
+        matrix_script:
+          - "opam pin list"
       max-parallel: 4
       fail-fast: false
     steps:
@@ -61,6 +63,21 @@ jobs:
         with:
           opam_file: './coq-demo.opam'
           custom_image: ${{ matrix.image }}
+          # Dummy test for https://github.com/coq-community/docker-coq-action/pull/33
+          before_install: |
+            startGroup "Install APT dependencies"
+              sudo apt-get update -y -q
+              sudo DEBIAN_FRONTEND=noninteractive apt-get install -y -q --no-install-recommends emacs
+            endGroup
+          before_script: |
+            startGroup "Preliminaries"
+              ${{matrix.matrix_script}}
+              opam config report
+            endGroup
+          after_script: |
+            startGroup "Post-test"
+              emacs --version
+            endGroup
 
 # Remark:
 # you may want to add the following badge to your README.md: