ci: add build and distribution pipeline with goreleaser and Gitea Actions

- .goreleaser.yaml: cross-compile linux/windows, Docker image, Gitea releases
- .gitea/workflows/ci.yml: verify build on every push/PR
- .gitea/workflows/release.yml: goreleaser + Docker on tag push
- Dockerfile: switch to debian:bookworm-slim, install steamcmd from Valve
- Dockerfile.goreleaser: minimal image used by goreleaser dockers
- docker-compose.yml: parameterize SERVERS_DIR via env var

.gitea/workflows/ci.yml

@@ -0,0 +1,20 @@
+name: CI
+on:
+  push:
+    branches:
+      - "**"
+  pull_request: {}
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-go@v5
+        with:
+          go-version: "1.25"
+      - uses: actions/setup-node@v4
+        with:
+          node-version: "22"
+      - run: cd backend && go build ./...
+      - run: cd frontend && npm ci && npm run build