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

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.25-alpine AS backend
+FROM golang:1.25-bookworm AS backend
 WORKDIR /src
 COPY backend/go.mod backend/go.sum ./
 RUN go mod download
@@ -12,9 +12,19 @@ RUN npm ci
 COPY frontend/ .
 RUN npm run build
 
-FROM alpine:3.21
-RUN apk add --no-cache ca-certificates tzdata
-RUN apk add --no-cache steamcmd libstdc++ libgcc || true
+FROM debian:bookworm-slim
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+      ca-certificates \
+      tzdata \
+      lib32gcc-s1 \
+      wget \
+      xz-utils && \
+    mkdir -p /steamcmd && \
+    wget -qO- https://steamcdn-a.akamaihd.net/client/installer/steamcmd_linux.tar.gz | \
+      tar xvz -C /steamcmd && \
+    ln -s /steamcmd/steamcmd.sh /usr/local/bin/steamcmd && \
+    rm -rf /var/lib/apt/lists/*
 
 COPY --from=backend /server /usr/local/bin/arma3-web-server
 COPY --from=frontend /src/dist /usr/share/arma3-web-server/frontend
@@ -22,12 +32,6 @@ COPY --from=frontend /src/dist /usr/share/arma3-web-server/frontend
 EXPOSE 8080
 VOLUME ["/data", "/servers"]
 
-# Expected mount structure under /servers:
-#   /servers/              ← SERVERFILE_DIR (arma3server_x64 binary, userconfig/)
-#   /servers/steamapps/workshop/content/107410/  ← workshop mods (downloaded via SteamCMD)
-#   /servers/mods/         ← MODS_DIR (@modname mod folders)
-#   /servers/cfg/          ← CFG_DIR (.cfg files)
-#   /servers/profiles/     ← PROFILES_DIR (logs, .rpt, server profile)
 ENV DATA_DIR=/data
 ENV SERVERFILE_DIR=/servers
 ENV MODS_DIR=/servers/mods