ci: Pinning container image to its SHA256 hash

Author: ecoussoux-ansys

.github/workflows/run_mapdl_tests.yml

@@ -22,7 +22,7 @@ jobs:
     name: Build tests
     runs-on: ubuntu-22.04
     container:
-      image: ghcr.io/ansys/mapdl:v22.2-ubuntu
+      image: ghcr.io/ansys/mapdl:v22.2-ubuntu@sha256:024c587f4a8190e99cc3f08a2dc231583032e784a8ef7d7659f8dd9748116697
       options: "-u=0:0 --entrypoint /bin/bash"
       credentials:
         username: ${{ secrets.GH_USERNAME }}