chore(docker): add Docker image build. Thanks @pexcn for the contributions!

Co-authored-by: Datong Sun <dndx@idndx.com>

docker/Dockerfile

@@ -0,0 +1,41 @@
+#
+# Dockerfile for phantun
+#
+
+#
+# Build stage
+#
+FROM rust:alpine AS builder
+
+COPY . /phantun
+
+RUN apk update \
+  && apk add --no-cache --virtual .build-deps musl-dev \
+  && cd phantun \
+  && cargo build --release \
+  && strip target/release/server target/release/client \
+  && install target/release/server /usr/local/bin/phantun-server \
+  && install target/release/client /usr/local/bin/phantun-client \
+  && cd - \
+  && rm -r phantun \
+  && apk del .build-deps \
+  && rm -rf /var/cache/apk/*
+
+#
+# Runtime stage
+#
+FROM alpine:3.16
+
+RUN apk update \
+  && apk add --no-cache iptables ip6tables tzdata \
+  && rm -rf /var/cache/apk/*
+
+COPY --from=builder /usr/local/bin/phantun-server /usr/local/bin/
+COPY --from=builder /usr/local/bin/phantun-client /usr/local/bin/
+COPY docker/phantun.sh /usr/local/bin/
+
+ENV USE_IPTABLES_NFT_BACKEND=0
+ENV RUST_LOG=INFO
+
+ENTRYPOINT ["phantun.sh"]
+CMD ["phantun-server", "--help"]