ci: add forgejo actions build workflow

.forgejo/workflows/build.yml

@@ -0,0 +1,46 @@
+name: Build and push image
+
+on:
+  push:
+    branches: [master]
+  schedule:
+    - cron: '0 4 * * *'   # daily, picks up LSIO base image updates
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: docker
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+          token: ${{ secrets.PUSH_TOKEN }}
+
+      - name: Merge upstream
+        run: |
+          git config user.name "forgejo-runner"
+          git config user.email "runner@forge.jannis.wtf"
+          git remote add upstream https://github.com/linuxserver/docker-plex
+          git fetch upstream
+          git merge upstream/master --no-edit
+          git push origin master
+
+      - name: Set up Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to Forgejo registry
+        uses: docker/login-action@v3
+        with:
+          registry: git.example.com
+          username: ${{ secrets.REGISTRY_USER }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: true
+          tags: |
+            forge.jannis.wtf/fej/plex-wireguard:latest
+            forge.jannis.wtf/fej/plex-wireguard:${{ github.sha }}