Add Dockerfile and GitHub Actions workflow for GHCR publishing

Co-authored-by: johndoe6345789 <224850594+johndoe6345789@users.noreply.github.com>

Dockerfile

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+# SparkOS Docker Image
+# Multi-stage build for minimal final image size
+
+# Build stage - use gcc image which has all build tools pre-installed
+FROM gcc:13-bookworm AS builder
+
+# Set working directory
+WORKDIR /build
+
+# Copy source files
+COPY src/ ./src/
+COPY Makefile .
+COPY scripts/ ./scripts/
+
+# Build the init system
+RUN make init
+
+# Runtime stage - use Alpine for minimal size
+FROM alpine:3.19
+
+# Note: Alpine includes busybox by default
+
+# Create minimal rootfs structure
+RUN mkdir -p /sparkos/rootfs/bin \
+    /sparkos/rootfs/sbin \
+    /sparkos/rootfs/etc \
+    /sparkos/rootfs/proc \
+    /sparkos/rootfs/sys \
+    /sparkos/rootfs/dev \
+    /sparkos/rootfs/tmp \
+    /sparkos/rootfs/usr/bin \
+    /sparkos/rootfs/usr/sbin \
+    /sparkos/rootfs/usr/lib \
+    /sparkos/rootfs/var/log \
+    /sparkos/rootfs/var/run \
+    /sparkos/rootfs/root \
+    /sparkos/rootfs/home/spark && \
+    chmod 1777 /sparkos/rootfs/tmp && \
+    chmod 700 /sparkos/rootfs/root && \
+    chmod 755 /sparkos/rootfs/home/spark
+
+# Copy built init binary from builder
+COPY --from=builder /build/init /sparkos/rootfs/sbin/init
+
+# Copy setup scripts
+COPY scripts/setup_rootfs.sh /sparkos/scripts/
+
+# Set up basic configuration files
+RUN echo "sparkos" > /sparkos/rootfs/etc/hostname && \
+    echo "127.0.0.1   localhost" > /sparkos/rootfs/etc/hosts && \
+    echo "127.0.1.1   sparkos" >> /sparkos/rootfs/etc/hosts && \
+    echo "root:x:0:0:root:/root:/bin/sh" > /sparkos/rootfs/etc/passwd && \
+    echo "spark:x:1000:1000:SparkOS User:/home/spark:/bin/sh" >> /sparkos/rootfs/etc/passwd && \
+    echo "root:x:0:" > /sparkos/rootfs/etc/group && \
+    echo "spark:x:1000:" >> /sparkos/rootfs/etc/group
+
+# Create a test entrypoint
+COPY <<'EOF' /sparkos/test.sh
+#!/bin/sh
+echo "SparkOS Docker Test Environment"
+echo "================================"
+echo ""
+echo "SparkOS init binary: /sparkos/rootfs/sbin/init"
+echo ""
+echo "Verifying init binary..."
+if [ -f /sparkos/rootfs/sbin/init ]; then
+    echo "✓ Init binary exists"
+    ls -lh /sparkos/rootfs/sbin/init
+    echo ""
+    echo "File type:"
+    file /sparkos/rootfs/sbin/init
+    echo ""
+    echo "Dependencies:"
+    ldd /sparkos/rootfs/sbin/init 2>&1 || echo "  Static binary (no dependencies)"
+else
+    echo "✗ Init binary not found!"
+    exit 1
+fi
+echo ""
+echo "Root filesystem structure:"
+ls -la /sparkos/rootfs/
+echo ""
+echo "SparkOS is ready for testing!"
+echo ""
+echo "To test the init system:"
+echo "  docker run --rm <image> /sparkos/rootfs/sbin/init --help"
+echo ""
+exec /bin/sh
+EOF
+
+RUN chmod +x /sparkos/test.sh
+
+WORKDIR /sparkos
+
+# Set entrypoint
+ENTRYPOINT ["/sparkos/test.sh"]