ci: encapsulate checks in Dockerfile, simplify CI to docker build

Per new policy: CI actions simply run 'docker build .'. The Dockerfile
now installs golangci-lint and runs 'make check' early in the build
process, so a successful docker build implies all checks pass.

- Dockerfile: add golangci-lint install and 'make check' before final build
- CI workflow: simplify to just 'docker build .' (no Go setup needed)
- Makefile targets unchanged

.gitea/workflows/check.yml

@@ -10,14 +10,7 @@ jobs:
   check:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13925f8d5 # v4
 
-      - uses: actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff # v5
-        with:
-          go-version-file: go.mod
-
-      - name: Install golangci-lint
-        run: go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@5d1e709b7be35cb2025444e19de266b056b7b7ee # v2.10.1
-
-      - name: Run make check
-        run: make check
+      - name: Build (includes make check)
+        run: docker build .

Dockerfile

@@ -20,7 +20,13 @@ RUN go mod download
 # Copy source code
 COPY . .
 
-# Build the binary
+# Install golangci-lint for checks
+RUN go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@5d1e709b7be35cb2025444e19de266b056b7b7ee
+
+# Run all checks (lint, vet, test, build)
+RUN make check
+
+# Build the final binary with version info
 RUN CGO_ENABLED=1 go build -v -o secret cmd/secret/main.go
 
 # Runtime stage