Add Gitea Actions CI workflow

Runs docker build . on push to main and pull requests.

.gitea/workflows/check.yml

@@ -0,0 +1,14 @@
+name: check
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: docker build .