ci: add manual Docker dev build workflow

Allows building and pushing a `dev`-tagged Docker image via
workflow_dispatch without affecting the official `latest` tag.

.github/workflows/docker-dev.yml

@@ -0,0 +1,77 @@
+name: Docker Dev
+
+on:
+  workflow_dispatch:
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        platform: [linux/amd64, linux/arm64]
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Build and push by digest
+        id: build
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          platforms: ${{ matrix.platform }}
+          outputs: type=image,name=${{ secrets.DOCKERHUB_USERNAME }}/dbx,push-by-digest=true,name-canonical=true,push=true
+          cache-from: type=gha,scope=${{ matrix.platform }}-dev
+          cache-to: type=gha,scope=${{ matrix.platform }}-dev,mode=max
+
+      - name: Export digest
+        run: |
+          mkdir -p /tmp/digests
+          digest="${{ steps.build.outputs.digest }}"
+          touch "/tmp/digests/${digest#sha256:}"
+
+      - name: Upload digest
+        uses: actions/upload-artifact@v4
+        with:
+          name: digests-${{ matrix.platform == 'linux/amd64' && 'amd64' || 'arm64' }}
+          path: /tmp/digests/*
+          if-no-files-found: error
+          retention-days: 1
+
+  docker-manifest:
+    needs: docker
+    runs-on: ubuntu-latest
+    steps:
+      - name: Download digests
+        uses: actions/download-artifact@v4
+        with:
+          path: /tmp/digests
+          pattern: digests-*
+          merge-multiple: true
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Create manifest list and push
+        working-directory: /tmp/digests
+        run: |
+          docker buildx imagetools create \
+            -t ${{ secrets.DOCKERHUB_USERNAME }}/dbx:dev \
+            $(printf '${{ secrets.DOCKERHUB_USERNAME }}/dbx@sha256:%s ' *)