Workflow to build pull requests

We want to run the build against pull request changes so we can ensure they do not break the build.

.github/workflows/gatsby-pr.yml

@@ -0,0 +1,49 @@
+# Workflow for building a Gatsby site
+#
+# To get started with Gatsby see: https://www.gatsbyjs.com/docs/quick-start/
+#
+name: Build Gatsby site (pull requests)
+
+on:
+  # Runs on pull requests targeting the default branch
+  pull_request:
+    branches: ["main"]
+
+# Default to bash
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  # Build job
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Detect package manager
+        id: detect-package-manager
+        run: |
+          if [ -f "${{ github.workspace }}/yarn.lock" ]; then
+            echo "manager=yarn" >> $GITHUB_OUTPUT
+            echo "command=install" >> $GITHUB_OUTPUT
+            exit 0
+          elif [ -f "${{ github.workspace }}/package.json" ]; then
+            echo "manager=npm" >> $GITHUB_OUTPUT
+            echo "command=ci" >> $GITHUB_OUTPUT
+            exit 0
+          else
+            echo "Unable to determine package manager"
+            exit 1
+          fi
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+          cache: ${{ steps.detect-package-manager.outputs.manager }}
+      - name: Install dependencies
+        run: ${{ steps.detect-package-manager.outputs.manager }} ${{ steps.detect-package-manager.outputs.command }}
+      - name: Build with Gatsby
+        env:
+          PREFIX_PATHS: 'true'
+        run: ${{ steps.detect-package-manager.outputs.manager }} run build