Move backend Dockerfile to repo root for git access

Place the backend Dockerfile at repo root as Dockerfile.backend so
the build context includes .git, giving git describe access for
version stamping. Fix .gitignore pattern to anchor /netwatch-server
so it does not exclude cmd/netwatch-server/. Remove .git from
.dockerignore. Update CI workflow and backend Makefile docker target.

.gitea/workflows/check.yml

@@ -7,4 +7,4 @@ jobs:
             # actions/checkout v4.2.2, 2026-02-22
             - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
             - run: docker build .
-            - run: docker build backend/
+            - run: docker build -f Dockerfile.backend .