Add GitHub workflow for building the OSTree repository

Also add a script to download and unpack the artifact on my server's
`public_html/`.

.github/workflows/build.yml

@@ -0,0 +1,32 @@
+name: build
+on:
+  schedule:
+    - cron: 0 2 * * 6
+  workflow_dispatch:
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container:
+      image: docker.io/fedora:latest
+      # Fix SELinux for the built OSTree: https://github.com/coreos/rpm-ostree/issues/1943
+      options: --privileged --security-opt label:disable
+    steps:
+      - name: Install dependencies
+        run: dnf install -y rpm-ostree selinux-policy selinux-policy-targeted policycoreutils
+
+      - name: Clone repository
+        uses: actions/checkout@v2
+
+      - name: Build OSTree
+        run: ./compose.sh
+
+      # upload-artifact@v2 has trouble with tens of thousands of files
+      - name: Create repo tarball
+        run: tar -czf /var/tmp/repo.tar.gz -C /var/tmp/repo .
+
+      - name: Create repository artifact
+        uses: actions/upload-artifact@v2
+        with:
+          name: repository
+          path: /var/tmp/repo.tar.gz
+          retention-days: 5