Remove hadolint and improve Dockerfile (#234)

Replace #190

See:

- https://gitea.com/gitea/act_runner/pulls/190#issuecomment-741196
- https://gitea.com/gitea/act_runner/pulls/208#issuecomment-741049

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/234
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: silverwind <silverwind@noreply.gitea.com>
Reviewed-by: delvh <dev.lh@web.de>

.gitea/workflows/test.yml

@@ -36,7 +36,3 @@ jobs:
         run: make build
       - name: test
         run: make test
-      - name: dockerfile lint check
-        uses: https://github.com/hadolint/hadolint-action@v3.1.0
-        with:
-          dockerfile: Dockerfile