Add a workflow to build this container

No way does this work.
I have no idea what I'm doing.

.github/workflows/build.yaml

@@ -0,0 +1,75 @@
+name: build-custom-silverblue
+on:
+  pull_request:
+  merge_group:
+  schedule:
+    - cron: '5 3 * * *'  # 3am-ish UTC everyday (timed against official fedora container pushes)
+  workflow_dispatch:
+
+jobs:
+  build_custom_silverblue:
+    name: main
+    runs-on: ubuntu-24.04
+    permissions:
+      contents: read
+      packages: write
+      id-token: write
+    strategy:
+      fail-fast: false
+
+    steps:
+      # Checkout push-to-registry action GitHub repository
+      - name: Checkout Push to Registry action
+        uses: actions/checkout@v4
+
+      # Build image using Buildah action
+      - name: Build Image
+        id: build_image
+        uses: redhat-actions/buildah-build@v2
+        with:
+          containerfiles: |
+            ./Containerfile
+          image: custom-silverblue
+          tags: |
+            ${{ github.sha }}
+          oci: false
+
+      - name: Push To GHCR
+        uses: redhat-actions/push-to-registry@v2
+        id: push
+        if: github.event_name != 'pull_request'
+        with:
+          image: ${{ steps.build_image.outputs.image }}
+          tags: ${{ steps.build_image.outputs.tags }}
+          registry: ghcr.io/${{ github.repository_owner }}
+          username: ${{ github.actor }}
+          password: ${{ github.token }}
+          extra-args: |
+            --disable-content-trust
+
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v3
+        if: github.event_name != 'pull_request'
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      # Sign container
+      - uses: sigstore/[email protected]
+        if: github.event_name != 'pull_request'
+
+      - name: Sign container image
+        if: github.event_name != 'pull_request'
+        run: |
+          cosign sign -y --key env://COSIGN_PRIVATE_KEY ${{ github.actor }}/${{ steps.build_image.outputs.image }}@${TAGS}
+        env:
+          TAGS: ${{ steps.push.outputs.outputs && fromJSON(steps.push.outputs.outputs).digest }}
+          COSIGN_EXPERIMENTAL: false
+          COSIGN_PRIVATE_KEY: ${{ secrets.SIGNING_SECRET }}
+
+      - name: Echo outputs
+        if: github.event_name != 'pull_request'
+        run: |
+          echo "${{ toJSON(steps.push.outputs) }}"
+