Add Dockerfile, .dockerignore, and Gitea Actions workflow

Dockerfile: golang:1.24-alpine (pinned by sha256), installs Node.js,
npm, golangci-lint (pinned by commit hash), and runs make check as a
build step so the image only builds on a green branch.

.gitea/workflows/check.yml: runs docker build . on push (actions/checkout
pinned by commit sha).

.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 .