Add Gitea Actions workflow for CI checks

Adds .gitea/workflows/check.yml that runs docker build on push,
matching the standard CI pattern used across all sneak/* repos.
The Dockerfile already runs make check (fmt-check, lint, test)
so a successful build implies all checks pass.

.gitea/workflows/check.yml

@@ -0,0 +1,9 @@
+name: check
+on: [push]
+jobs:
+    check:
+        runs-on: ubuntu-latest
+        steps:
+            # actions/checkout v4.2.2, 2026-03-16
+            - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
+            - run: docker build .