[feat](build): add versioned Docker image build script and version injection

- Introduced `tools/build_server_image.sh` for building versioned server images with support for multi-arch builds.
- Added `VERSION` injection via `-ldflags` in Dockerfile and Go binaries for both server and client.
- Updated workflows and Makefile to ensure consistent version tagging during builds.

tools/build_server_image.sh

@@ -0,0 +1,43 @@
+. #!/usr/bin/env bash
+
+set -euo pipefail
+
+# Build hop-gate server image from Dockerfile.server.
+# VERSION is derived from current git commit (7-char SHA).
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 && pwd)"
+REPO_ROOT="${SCRIPT_DIR}/.."
+cd "${REPO_ROOT}"
+
+VERSION="$(git rev-parse --short=7 HEAD 2>/dev/null || echo dev)"
+
+# Default image name; can be overridden by first argument.
+# Usage:
+#   ./tools/build_server_image.sh                 # builds ghcr.io/dalbodeule/hop-gate:<hash> and :latest
+#   ./tools/build_server_image.sh my/image/name   # builds my/image/name:<hash> and :latest
+IMAGE_NAME="${1:-ghcr.io/dalbodeule/hop-gate}"
+
+echo "Building hop-gate server image"
+echo "  context : ${REPO_ROOT}"
+echo "  image   : ${IMAGE_NAME}:${VERSION}"
+echo "  version : ${VERSION}"
+
+# Use docker buildx if available; fallback to docker build.
+if command -v docker >/dev/null 2>&1 && docker buildx version >/dev/null 2>&1; then
+  BUILD_CMD=(docker buildx build)
+else
+  BUILD_CMD=(docker build)
+fi
+
+# Optional environment variables:
+#   PLATFORM=linux/amd64,linux/arm64   (for buildx)
+#   PUSH=1                              (for buildx --push)
+
+"${BUILD_CMD[@]}" \
+  ${PLATFORM:+--platform "${PLATFORM}"} \
+  -f Dockerfile.server \
+  --build-arg VERSION="${VERSION}" \
+  -t "${IMAGE_NAME}:${VERSION}" \
+  -t "${IMAGE_NAME}:latest" \
+  ${PUSH:+--push} \
+  .