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.goreleaser

@@ -0,0 +1,31 @@
+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 arma3-web-server /usr/local/bin/arma3-web-server
+COPY frontend/dist /usr/share/arma3-web-server/frontend
+
+EXPOSE 8080
+VOLUME ["/data", "/servers"]
+
+ENV DATA_DIR=/data
+ENV SERVERFILE_DIR=/servers
+ENV MODS_DIR=/servers/mods
+ENV CFG_DIR=/servers/cfg
+ENV PROFILES_DIR=/servers/profiles
+ENV LISTEN=:8080
+ENV FRONTEND_DIR=/usr/share/arma3-web-server/frontend
+ENV GIN_MODE=release
+
+ENTRYPOINT ["arma3-web-server"]