CI: run the built docker container and mount the repo

this makes it easier to debug CI things locally

.gitlab-ci/build.sh

@@ -3,4 +3,7 @@
 set -e
 TAG="registry.gitlab.tugraz.at/dbp/web-components/toolkit/main:v2"
 sudo docker build --tag "${TAG}" --file "Dockerfile" .
+sudo docker run --rm \
+    --volume "$(pwd)/..:/home/user/app" --workdir "/home/user/app" \
+    --tty --interactive "${TAG}" bash
 echo "Now run: sudo docker push '$TAG'"