build: use Alpine Linux for minimal Docker image (~9MB)

Inspired by PR #3 (ferchault). Switch from Ubuntu 24.04 to Alpine 3.21
for both build and runtime stages. Static linking eliminates runtime
dependencies, producing a ~9MB image instead of ~80MB.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: steinbeck

Dockerfile

@@ -1,33 +1,25 @@
-FROM ubuntu:24.04 AS builder
+FROM alpine:3.21 AS builder
 
-RUN apt-get update && \
-    apt-get -y install curl gcc make zlib1g-dev && \
-    apt-get -y clean && \
-    rm -rf /var/lib/apt/lists/*
+RUN apk add --no-cache build-base curl zlib-dev
 
 WORKDIR /build
 
 # Download and build nauty
 RUN curl -L -o nauty2_9_3.tar.gz https://users.cecs.anu.edu.au/~bdm/nauty/nauty2_9_3.tar.gz \
     && tar xzf nauty2_9_3.tar.gz \
     && cd nauty2_9_3 \
-    && ./configure && make
+    && ./configure && make -j4
 
-# Copy source and build surge
+# Copy source and build surge (static linking for minimal runtime image)
 COPY src/ /build/src/
 WORKDIR /build/src
 RUN make surge \
     NAUTY=/build/nauty2_9_3 \
     NAUTYLIB=/build/nauty2_9_3 \
-    CCOPT="-O3"
+    CCOPT="-O3 -static"
 
-# Runtime image
-FROM ubuntu:24.04
-
-RUN apt-get update && \
-    apt-get -y install zlib1g && \
-    apt-get -y clean && \
-    rm -rf /var/lib/apt/lists/*
+# Minimal runtime image (~9MB)
+FROM alpine:3.21
 
 COPY --from=builder /build/src/surge /usr/local/bin/surge