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

.dockerignore

@@ -0,0 +1,19 @@
+node_modules/
+frontend/node_modules/
+frontend/dist/
+data/
+serverfiles/
+.git/
+.gitignore
+*.log
+*.rpt
+.DS_Store
+.env
+modlist-example.html
+Start-Server-kp.bat
+PLAN.md
+CODEBASE.md
+ARMA3.md
+.gitea/
+.goreleaser.yaml
+.dockerignore