Add hash-pinned Dockerfile, .dockerignore, and Gitea CI workflow

Multistage build per policy: a fail-fast lint stage on the pinned
golangci-lint image runs fmt-check and lint, the builder stage reuses
its linter binary (which also forces stage ordering), runs make check,
and builds; the runtime stage is pinned alpine with just the binary.
CI runs docker build . on push with the checkout action pinned by
commit SHA. All image references pinned by sha256 digest with
version/date comments.

.gitea/workflows/check.yml

@@ -0,0 +1,9 @@
+name: check
+on: [push]
+jobs:
+    check:
+        runs-on: ubuntu-latest
+        steps:
+            # actions/checkout v4.2.2, 2026-02-22
+            - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
+            - run: docker build .