Added Docker multi-stage scratch build to slim down container size fr…

…om 743MB to about 4MB.

Dockerfile

@@ -1,10 +1,31 @@
-FROM alpine:latest
+FROM alpine:latest as builder
 
-RUN apk update
-RUN apk add gcc make ca-certificates git libc-dev linux-headers openssl perl zlib-dev
+# Ensure no packages are cached before we try to do an update.
+RUN apk cache clean 2> /dev/null || exit 0
 
+RUN apk update && apk add gcc make ca-certificates git libc-dev linux-headers openssl perl zlib-dev
 RUN update-ca-certificates
+
 ADD . builddir
-RUN cd builddir; make static; cp /builddir/sslscan /usr/local/bin
 
-ENTRYPOINT ["sslscan"]
+# Make a static build of sslscan, then strip it of debugging symbols.
+RUN cd builddir && make static
+RUN strip --strip-all /builddir/sslscan
+
+# Print the output of ldd so we can see what dynamic libraries that sslscan is still dependent upon.
+RUN echo "ldd output:" && ldd /builddir/sslscan
+RUN echo "ls -al output:" && ls -al /builddir/sslscan
+
+
+# Start with an empty container for our final build.
+FROM scratch
+
+# Copy over the sslscan executable from the intermediate build container, along with the dynamic libraries it is dependent upon (see output of ldd, above).
+COPY --from=builder /builddir/sslscan /sslscan
+COPY --from=builder /lib/libz.so.1 /lib/libz.so.1
+COPY --from=builder /lib/ld-musl-x86_64.so.1 /lib/ld-musl-x86_64.so.1
+
+# Drop root privileges.
+USER 65535:65535
+
+ENTRYPOINT ["/sslscan"]