CI: Fix Arch package build step

Fixes #72.

- Create and use a 'builder' user instead of 'nobody'
- Add --privileged to 'docker run'

Restores some changes from b25d4de

.github/workflows/main.yml

@@ -6,7 +6,7 @@ jobs:
   build:
     # Perform a generic build in case of any critical errors
     name: Build
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
     # Checks out under $GITHUB_WORKSPACE
     - name: Checkout
@@ -80,6 +80,14 @@ jobs:
         source PKGBUILD
         echo "GTK_VERSION=${_gtkver}" >> $GITHUB_ENV
 
+    - name: Cache Pacman Packages
+      uses: actions/cache@v2
+      env:
+        cache-name: pacman-cache
+      with:
+        path: ~/pacman-cache
+        key: pacman-cache
+
     - name: Cache GTK Source
       uses: actions/cache@v2
       id: gtksrc
@@ -92,20 +100,33 @@ jobs:
         cp PKGBUILD PKGBUILD.before
         md5sum PKGBUILD >> PKGBUILD.md5
 
-    - name: Validate Package
+    - name: Start Arch Linux
       run: |
-        # makepkg cannot run as root
-        sudo chown -R nobody .
-
-        # Start Arch using Docker
-        docker run -d -t -v $GITHUB_WORKSPACE:/workspace --name archlinux archlinux:base-devel
+        # Start and update Arch Linux within a Docker container
+        docker run --privileged -d -t -v ~/pacman-cache/pkg:/var/cache/pacman/pkg -v $GITHUB_WORKSPACE:/workspace --name archlinux archlinux:base-devel
         docker exec archlinux pacman -Sy --noconfirm namcap
 
+        # Add user
+        docker exec archlinux useradd builder
+        docker exec archlinux /bin/bash -c "echo 'builder ALL=(ALL:ALL) NOPASSWD:ALL' >> /etc/sudoers"
+        docker exec archlinux chown -R builder /workspace
+
+        # Enable 32-bit lib support
+        docker exec archlinux /bin/bash -c "echo -e '\n[multilib]\nInclude = /etc/pacman.d/mirrorlist' >> /etc/pacman.conf"
+        docker exec archlinux pacman -Syu --noconfirm
+
+    - name: Validate Package
+      run: |
         # Validate the PKGBUILD is the same and free from errors
-        docker exec --workdir=/workspace --user nobody archlinux makepkg --geninteg
-        docker exec --workdir=/workspace --user nobody archlinux namcap -i PKGBUILD
+        docker exec --workdir=/workspace --user builder archlinux makepkg --geninteg
+        docker exec --workdir=/workspace --user builder archlinux namcap -i PKGBUILD
 
-        # Stop containers
+    - name: Build Arch Package
+      run: |
+        docker exec --workdir=/workspace --user builder archlinux makepkg -s --noconfirm
+
+    - name: Stop Arch Linux
+      run: |
         docker stop archlinux -t 1
         docker rm archlinux