ci/cd: fix IPv6 requests not working with fetch

- Fixes Docker build checks failing because `localhost` is resolving to
  IPv6.
- Fixes external URL checks failing because external URLs are resolving
  to IPv6.

.github/actions/force-ipv4/action.yml

@@ -0,0 +1,12 @@
+inputs:
+  project-root:
+    required: false
+    default: '.'
+runs:
+  using: composite
+  steps:
+    -
+      name: Run prefer IPv4 script
+      shell: bash
+      run: ./.github/actions/force-ipv4/force-ipv4.sh
+      working-directory: ${{ inputs.project-root }}

.github/actions/force-ipv4/force-ipv4.sh

@@ -0,0 +1,52 @@
+#!/usr/bin/env bash
+
+main() {
+  if is_linux; then
+    echo 'Configuring Linux...'
+    # prefer_ipv4_on_linux TODO: Not working
+    disable_ipv6_on_linux
+  elif is_macos; then
+    echo 'Configuring macOS...'
+    disable_ipv6_on_macos
+  fi
+}
+
+is_linux() {
+  [[ "$(uname -s)" == "Linux" ]]
+}
+
+is_macos() {
+  [[ "$(uname -s)" == "Darwin" ]]
+}
+
+disable_ipv6_on_linux() {
+  sudo sysctl -w net.ipv6.conf.all.disable_ipv6=1
+  sudo sysctl -w net.ipv6.conf.default.disable_ipv6=1
+  sudo sysctl -p
+
+  local -r sysctl_conf="/etc/sysctl.conf"
+  echo "net.ipv6.conf.all.disable_ipv6 = 1" | sudo tee -a "$sysctl_conf" > /dev/null
+  echo "net.ipv6.conf.default.disable_ipv6 = 1" | sudo tee -a "$sysctl_conf" > /dev/null
+  echo "net.ipv6.conf.lo.disable_ipv6 = 1" | sudo tee -a "$sysctl_conf" > /dev/null
+}
+
+prefer_ipv4_on_linux() {
+  local -r gai_conf="/etc/gai.conf"
+  if [ ! -f "$gai_conf" ]; then
+      echo "Creating $gai_conf since it doesn't exist..."
+      touch "$gai_conf"
+  fi
+  echo "precedence ::ffff:0:0/96  100" | sudo tee -a "$gai_conf" > /dev/null
+  echo "Configuration complete."
+}
+
+disable_ipv6_on_macos() {
+  networksetup -listallnetworkservices \
+    | tail -n +2 \
+    | while IFS= read -r interface; do
+        echo "Disabling IPv6 on: $interface"
+        networksetup -setv6off "$interface"
+      done
+}
+
+main

.github/workflows/checks.build.yaml

@@ -95,6 +95,9 @@ jobs:
       -
         name: Run Docker image on port 8080
         run: docker run -d -p 8080:80 --rm --name privacy.sexy undergroundwires/privacy.sexy:latest
+      -
+        name: Prefer IPv4 # GitHub runners do not support IPv6 and requests fail when they try to connect to IPv6 addresses by default.
+        uses: ./.github/actions/force-ipv4
       -
         name: Check server is up and returns HTTP 200
         run: >-

.github/workflows/checks.external-urls.yaml

@@ -17,6 +17,9 @@ jobs:
       -
         name: Install dependencies
         uses: ./.github/actions/npm-install-dependencies
+      -
+        name: Force IPv4 # GitHub runners do not support IPv6 and requests fail when they try to connect to IPv6 addresses by default.
+        uses: ./.github/actions/force-ipv4
       -
         name: Test
         run: npm run check:external-urls