Set version of masscan to checkout and build.

Dockerfile

 FROM buildpack-deps:stretch-scm as build
+ARG VERSION=1.0.5
 ENV DEBIAN_FRONTEND noninteractive
 RUN apt-get update && \
     apt-get install -y \
@@ -6,11 +7,13 @@ RUN apt-get update && \
         clang \
         libpcap-dev \
     && \
-    git clone https://github.com/robertdavidgraham/masscan.git && \
+    git clone https://github.com/robertdavidgraham/masscan.git --branch ${VERSION} && \
     cd masscan && \
     make -j
 
 FROM debian:stretch-slim
+ARG VERSION=1.0.5
+LABEL MASSCAN_VERSION=${VERSION}
 RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get install -y libpcap0.8 && \
     rm -rf /tmp/* /var/tmp/* /var/lib/apt/lists/* /var/cache/apt/archives/*