Update Dockerfile to use Alpine, and tweak docs

Author: rbsec

.dockerignore

@@ -0,0 +1 @@
+openssl

Dockerfile

@@ -1,15 +1,10 @@
-FROM ubuntu:18.04
+FROM alpine:latest
 
-# Install nginx for some tests.
-# Install strace for potential debugging, and rsyslog to enable system log gathering.
-RUN apt-get update 
-RUN apt-get install -y  ca-certificates build-essential git zlib1g-dev openssl 
-RUN apt-get clean 
+RUN apk update
+RUN apk add gcc make ca-certificates git libc-dev linux-headers openssl perl zlib-dev
 
 RUN update-ca-certificates
-ADD . builddir 
+ADD . builddir
 RUN cd builddir; make static; cp /builddir/sslscan /usr/local/bin
 
 ENTRYPOINT ["sslscan"]
-
-EXPOSE 443

Makefile

@@ -155,7 +155,7 @@ openssl/libcrypto.a: openssl/Makefile
 static: openssl/libcrypto.a
 	$(MAKE) -j $(NUM_PROCS) sslscan STATIC_BUILD=TRUE
 
-build-image:
+docker:
 	docker build -t sslscan:sslscan .
 
 test:	static

README.md

@@ -91,13 +91,18 @@ This will clone the [OpenSSL repository](https://github.com/openssl/openssl), an
 **Please note:** Out of the box, OpenSSL cannot compiled with `clang` without further customization (which is not done by the provided `Makefile`). For more information on this, see [Modifying Build Settings](http://wiki.openssl.org/index.php/Compilation_and_Installation#Modifying_Build_Settings) in the OpenSSL wiki.
 
 You can verify whether you have a statically linked OpenSSL version, by checking whether the version listed by `sslscan --version` has the `-static` suffix.
-### Building with docker
 
-Install docker on your linux.
+### Building with Docker
+
+Ensure that you local Docker installation is functional, and the build the container with:
+
+    make docker
+
+Or manually with:
 
    docker build -t sslscan:sslscan .
 
-to use it
+You can then run sslscan with:
 
    docker run --rm -ti sslscan:sslscan --help