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

docker-compose.yml

@@ -6,8 +6,7 @@ services:
       - "8080:8080"
     volumes:
       - ./data:/data
-      # /servers should contain the Arma 3 server install (binary + steamapps/workshop for mods)
-      - /path/to/your/servers:/servers
+      - ${SERVERS_DIR:-./serverfiles}:/servers
     environment:
       - DATA_DIR=/data
       - SERVERFILE_DIR=/servers