Added job to build nightly container (#24595)

.github/workflows/build-nightly.yml

@@ -0,0 +1,52 @@
+name: Build nightly container image
+on:
+  schedule:
+    - cron: '0 2 * * *' # run at 2 AM UTC
+permissions:
+  contents: read
+  packages: write
+
+jobs:
+  build-nightly-image:
+    runs-on: ubuntu-latest
+
+    concurrency:
+      group: ${{ github.workflow }}-${{ github.ref }}
+      cancel-in-progress: true
+
+    steps:
+      - uses: actions/checkout@v3
+      - uses: hadolint/hadolint-action@v3.1.0
+      - uses: docker/setup-qemu-action@v2
+      - uses: docker/setup-buildx-action@v2
+
+      - name: Log in to the Github Container registry
+        uses: docker/login-action@v2
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - uses: docker/metadata-action@v4
+        id: meta
+        with:
+          images: |
+            ghcr.io/mastodon/mastodon
+          flavor: |
+            latest=auto
+          tags: |
+            type=raw,value=nightly
+          labels: |
+            org.opencontainers.image.description=Nightly build image used for testing purposes
+
+      - uses: docker/build-push-action@v4
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          provenance: false
+          builder: ${{ steps.buildx.outputs.name }}
+          push: ${{ github.repository == 'mastodon/mastodon' && github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max