Add docker logout call to docker release action.

`docker login` has a side effect of storing the
credentials passed to it in the `.docker/config.json`
which can be a security concern. This ameliorates
the problem for us.

Author: NlightNFotis

.github/workflows/release-packages.yaml

@@ -226,3 +226,6 @@ jobs:
         run: |
           echo ${{ secrets.DOCKERHUB_ACCESS_DB_CI_CPROVER }} | docker login --username=dbcicprover --password-stdin
           docker image push "$IMAGE_TAG"
+          # For security reasons remove stored login credentials from
+          # configuration file they are stored at by docker login.
+          docker logout