Test unofficial tarball when new tags are pushed

This so we can always test that the final tarball as currently pushed at
domjudge.org passes some basic tests.

We had this as manual job on GitLab in a slightly different form but
making sure this is done explicit is a good thing IMO.

The next step would be to do this for every push to main so before a new
tag is pushed we know that all simple steps which we don't use ourselves
do work.

.github/workflows/build-tarball-release.yml

@@ -0,0 +1,73 @@
+name: 'Build example tarball'
+
+on:
+  push:
+    tags:
+      - '[0-9]+.[0-9]+.[0-9]+'
+
+jobs:
+  create-unofficial-tarball:
+    if: github.repository == 'domjudge/domjudge'
+    name: Build example tarball (Unofficial)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Download release script
+        run: wget https://raw.githubusercontent.com/DOMjudge/domjudge-scripts/main/make_release.sh
+
+      - name: Bring apt in a good state
+        run: sudo apt update
+
+      - name: Install development needed packages
+        run: |
+          sudo apt install -y autoconf automake bats \
+                              python3-sphinx python3-sphinx-rtd-theme rst2pdf fontconfig python3-yaml \
+                              latexmk texlive-latex-recommended texlive-latex-extra tex-gyre
+
+      - name: Create release tarball
+        run: |
+          sh ./make_release.sh ${{ github.ref_name }}
+
+      - name: Install our new version dependencies
+        run: |
+          sudo apt install -y acl zip unzip mariadb-server apache2 php php-fpm php-gd php-cli php-intl php-mbstring \
+                              php-mysql php-curl php-json php-xml php-zip composer ntp \
+                              make pkg-config sudo debootstrap libcgroup-dev php-cli php-curl php-json php-xml \
+                              php-zip lsof procps
+          
+      - name: Unpack the tarball again
+        run: |
+          for directory in build all docs domserver judgehost; do
+            mkdir /tmp/${directory}
+            tar zxvf domjudge-${{ github.ref_name }}.tar.gz --directory /tmp/${directory}
+          done
+
+      - name: Configure and install in default setting
+        run: |
+          for directory in build all docs domserver judgehost; do
+            cd /tmp/${directory}/domjudge-${{ github.ref_name }}
+            ./configure
+            make ${directory}
+            if [ "$directory" != "docs" ] && [ "$directory" != "judgehost" ]; then
+              sudo make install-domserver
+            fi
+            if [ "$directory" != "docs" ] && [ "$directory" != "domserver" ]; then
+              sudo make install-judgehost
+            fi
+            if [ "$directory" = "docs" ] || [ "$directory" = "all" ]; then
+              sudo make install-docs
+            fi
+            make clean
+            make distclean
+          done
+
+      - name: "See the new_release_howto.txt (L28/domjudge-scripts) for the next step."
+        run: true
+
+      - uses: actions/upload-artifact@v4
+        with:
+          name: unofficial-tarball-${{ github.ref_name }}
+          path: domjudge-${{ github.ref_name }}.tar.gz
+          retention-days: 2
+