ci: add VM test-demo workflow

.github/workflows/test-demo.sh

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+#!/usr/bin/env bash
+
+# nix build .#overview
+# podman run --privileged \
+#   --volume ./result/project/Cryptpad/default.nix:/default.nix \
+#   --volume .github/workflows/test-demo.sh:/test-demo.sh <DISTRO> /bin/bash \
+#   -c "bash /test-demo.sh <DISTRO>"
+
+set -euo pipefail
+
+DISTRO="$1"
+# shellcheck disable=SC2089,2026
+NIX_CONFIG='substituters = https://cache.nixos.org/ https://ngi.cachix.org/'$'\n''trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= ngi.cachix.org-1:n+CAL72ROC3qQuLxIHpV+Tw5t42WhXmMhprAGkRSrOw='
+export NIX_CONFIG
+
+
+echo -e "\n-> Installing Nix ..."
+# Debian/Ubuntu
+if echo "$DISTRO" | grep --quiet "debian\|ubuntu"; then
+    apt update
+    apt install --yes curl git jq nix
+# Archlinux
+elif echo "$DISTRO" | grep --quiet archlinux; then
+    pacman --sync --refresh --noconfirm curl git jq nix
+# Other
+else
+    echo "ERROR: Unknown distro. Exiting ..."
+    exit 1
+fi
+
+echo -e "\n-> Nix version ..."
+function fver { printf '%d%02d%02d' "${1}" "${2:-0}" "${3:-0}"; }
+NIX_VERSION=$(fver $(nix --version | grep -oP '([0-9]+\.?)+' | sed  's/\./ /g'))
+echo "Nix version: $NIX_VERSION"
+
+echo -e "\n-> Building VM ..."
+# Nix versions < 2.24 don't work for our use case due to regression in
+# closureInfo.
+# https://github.com/NixOS/nix/issues/6820
+if [ "$NIX_VERSION" -ge 22400 ]; then
+    echo "Using Nix installed by Linux package manager"
+    nix-build /default.nix
+else
+    echo "Using Nix from Nixpkgs unstable"
+
+    nixpkgs_revision=$(
+        nix-instantiate --eval --attr sources.nixpkgs.rev https://github.com/ngi-nix/ngipkgs/archive/master.tar.gz \
+        | jq --raw-output
+    )
+    NIXPKGS="https://github.com/NixOS/nixpkgs/archive/$nixpkgs_revision.tar.gz"
+
+    nix-shell --include nixpkgs="$NIXPKGS" --packages nix --run "nix-build /default.nix"
+fi
+
+echo -e "\n-> Launching VM ..."
+./result &
+
+echo -e "\n-> Running test ..."
+curl --retry 10 --retry-all-errors --fail localhost:9000 | grep CryptPad

.github/workflows/test-demo.yaml

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+name: Test VM demo
+
+permissions: {}
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+  workflow_dispatch:
+
+jobs:
+  test:
+    strategy:
+      fail-fast: false
+      matrix:
+        distro:
+          - "archlinux:latest"
+          - "debian:12"
+          - "debian:unstable"
+          - "ubuntu:24.04"
+          - "ubuntu:24.10"
+          - "ubuntu:devel"
+
+    runs-on: ubuntu-latest
+    steps:
+      - uses: 'actions/checkout@v4'
+        with: { persist-credentials: false }
+
+      - uses: DeterminateSystems/nix-installer-action@21a544727d0c62386e78b4befe52d19ad12692e3  #v17
+
+      - name: Build projects overview
+        run: nix build .#overview
+
+      - name: Run and test VM
+        run: >
+          docker run
+          --privileged
+          --volume ./result/project/Cryptpad/default.nix:/default.nix
+          --volume "$(pwd)/.github/workflows/test-demo.sh:/test-demo.sh"
+          ${{ matrix.distro }}
+          /bin/bash -c "bash /test-demo.sh ${{ matrix.distro }}"