Add Arch Linux GitHub Actions workflow

- Add manual workflow for building and testing on Arch Linux
- Uses Docker container with Dockerfile.archlinux
- Triggered via workflow_dispatch for on-demand testing

Author: mfisher31

.github/workflows/archlinux.yml

@@ -0,0 +1,30 @@
+name: Arch Linux Build
+
+on:
+  workflow_dispatch:
+
+jobs:
+  build-archlinux:
+    runs-on: ubuntu-latest
+    
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
+      with:
+        submodules: recursive
+    
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v3
+    
+    - name: Build Arch Linux Docker image
+      run: docker build -f Dockerfile.archlinux -t element:archlinux .
+    
+    - name: Build and test Element in Arch Linux container
+      run: |
+        docker run --rm --user $(id -u):$(id -g) -v $(pwd):/workspace element:archlinux bash -c "
+          git config --global --add safe.directory /workspace && \
+          git submodule update --init --recursive && \
+          cmake -B build-arch -G Ninja -DCMAKE_BUILD_TYPE=Release -DELEMENT_BUILD_PLUGINS=ON -DELEMENT_BUILD_TESTS=ON && \
+          cmake --build build-arch && \
+          ctest --test-dir build-arch --output-on-failure
+        "