ci: run syntax-check in container (fix missing git)

.gitea/workflows/ci.yml

@@ -10,9 +10,12 @@ on:
 jobs:
   syntax-check:
     runs-on: dotfiles
+    container:
+      image: python:3-slim
     steps:
       - name: Checkout
         run: |
+          apt-get update -qq && apt-get install -y -qq git >/dev/null
           git clone --depth 1 --branch "${GITHUB_REF_NAME}" \
             "https://oauth2:${{ github.token }}@${GITHUB_SERVER_URL#https://}/${GITHUB_REPOSITORY}.git" .