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

.goreleaser.yaml

@@ -0,0 +1,52 @@
+version: 2
+
+project_name: arma3-web-server
+
+before:
+  hooks:
+    - cd frontend && npm ci && npm run build
+
+builds:
+  - id: server
+    dir: backend
+    main: ./cmd/server/
+    binary: arma3-web-server
+    goos:
+      - linux
+      - windows
+    goarch:
+      - amd64
+    env:
+      - CGO_ENABLED=0
+
+archives:
+  - id: default
+    name_template: >-
+      {{ .ProjectName }}_{{ .Tag }}_{{ .Os }}_{{ .Arch }}
+    files:
+      - frontend/dist/**/*
+
+dockers:
+  - ids:
+      - server
+    goos: linux
+    goarch: amd64
+    image_templates:
+      - "{{ .Env.REGISTRY }}/{{ .Env.REPOSITORY_OWNER }}/arma3-web-server:{{ .Tag }}"
+      - "{{ .Env.REGISTRY }}/{{ .Env.REPOSITORY_OWNER }}/arma3-web-server:latest"
+    dockerfile: Dockerfile.goreleaser
+    extra_files:
+      - frontend/dist
+
+checksum:
+  name_template: "checksums.txt"
+
+snapshot:
+  name_template: "{{ incpatch .Version }}-next"
+
+release:
+  gitea:
+    owner: "{{ .Env.REPOSITORY_OWNER }}"
+    name: arma3-web-server
+  draft: false
+  prerelease: auto