feat: add Docker image and GHCR workflow (#3)

Dockerfile

@@ -0,0 +1,43 @@
+# syntax=docker/dockerfile:1.7
+
+FROM --platform=$TARGETPLATFORM golang:1.25.0-bookworm AS build
+
+ARG VERSION=dev
+
+WORKDIR /src
+
+COPY go.mod go.sum ./
+RUN --mount=type=cache,target=/go/pkg/mod \
+    go mod download
+
+COPY . .
+
+RUN --mount=type=cache,target=/go/pkg/mod \
+    --mount=type=cache,target=/root/.cache/go-build \
+    go build -trimpath -ldflags="-s -w -checklinkname=0 -X github.com/fumiama/WireGold/config.Version=${VERSION}" \
+    -o /out/wg .
+
+FROM debian:bookworm-slim
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+LABEL org.opencontainers.image.title="WireGold" \
+      org.opencontainers.image.description="Container image for WireGold, a pure-Go Layer 3 VPN inspired by WireGuard." \
+      org.opencontainers.image.source="https://github.com/fumiama/WireGold"
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends ca-certificates iproute2 tini \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /config
+
+COPY --from=build /out/wg /usr/local/bin/wg
+COPY docker-entrypoint.sh /usr/local/bin/docker-entrypoint.sh
+
+RUN chmod +x /usr/local/bin/wg /usr/local/bin/docker-entrypoint.sh \
+    && mkdir -p /config
+
+VOLUME ["/config"]
+
+ENTRYPOINT ["/usr/bin/tini", "--", "/usr/local/bin/docker-entrypoint.sh"]
+CMD ["-c", "/config/config.yaml"]