meta: add forgejo workflow

.forgejo/workflows/build.yaml

@@ -0,0 +1,22 @@
+on:
+  push:
+
+jobs:
+  build-x86_64: &build
+    container: registry.git.ptrc.gay/ptrcnull/ptrcports-builder/x86_64:latest
+    steps:
+      - uses: https://code.forgejo.org/actions/checkout@v4
+      - uses: https://code.forgejo.org/actions/cache@v4
+        with:
+          path: ~/repodest
+          key: ${{ runner.os }}-ptrcports
+
+      - name: Set up secrets
+        run: |
+          echo "${{ secrets.SSH_KEY }}" | base64 -d > /tmp/ssh_key
+          echo "${{ secrets.PACKAGER_PRIVKEY }}" | base64 -d > /tmp/packager_key
+      - name: Do the build
+        env:
+          PRIVATE_KEY: /tmp/ssh_key
+          PACKAGER_PRIVKEY: /tmp/packager_key
+        run: cd alpine && do-the-build