feat: add build job

- add build job that runs `make plan`

.gitea/workflows/build.yaml

@@ -0,0 +1,32 @@
+---
+name: Build
+
+on:
+  pull_request:
+
+jobs:
+  build:
+    runs-on: almalinux-8
+    container:
+      image: git.unkin.net/unkin/almalinux9-actionsdind:latest
+      options: --privileged
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+
+      - name: Fetch master branch
+        run: |
+          git fetch origin master:master
+
+      - name: Show changed files
+        run: |
+          git diff --name-only master
+
+      - name: Run OpenTofu Plan
+        env:
+          VAULT_ROLEID: ${{ secrets.TERRAFORM_INCUS_VAULT_ROLEID }}
+        run: |
+          make plan