fix: split Dockerfile with pinned images and add CI workflow

- Pin golangci-lint to v1.64.8 by sha256 digest
- Pin golang to 1.22.12 by sha256 digest
- Lint stage runs make fmt-check + make lint
- Test stage runs make test with dependency on lint stage
- Remove redundant final stage (library has no binary)
- Add fmt-check, check, hooks targets to Makefile
- Add .gitea/workflows/check.yml for CI

closes #9

.gitea/workflows/check.yml

@@ -0,0 +1,13 @@
+name: check
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: docker build .