let's a manual build option to mirror upstream's new manual build

.github/workflows/manual_build.yml

@@ -0,0 +1,25 @@
+# Builds a new image from Valhalla's "manually_triggered_build" image
+
+name: manual image build
+on:
+  workflow_dispatch
+
+jobs:
+  publish:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out the repo
+        uses: actions/checkout@v3
+
+      - name: Log in to GitHub Docker Registry
+        uses: docker/login-action@v2
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build latest image
+        run: |
+          sed -i "s/:latest/:manually_triggered_build/g" Dockerfile
+          docker build -t ghcr.io/${{ github.repository }}/valhalla:manually_triggered_build .
+          docker push ghcr.io/${{ github.repository }}/valhalla:manually_triggered_build