hadolint recommendations

Dockerfile

@@ -1,4 +1,7 @@
-FROM ubuntu:latest
+# LTS Image
+FROM ubuntu:18.04
+
+LABEL maintainer="github.google-sre-ebook@captnemo.in"
 
 ARG DEBIAN_FRONTEND="noninteractive"
 
@@ -15,7 +18,9 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     wget \
     zlib1g-dev \
     && gem install bundler --no-ri --no-rdoc \
-    && bundle install
+    && bundle install \
+    && apt-get clean \
+    && rm -rf /var/lib/apt/lists/*
 
 ENTRYPOINT ["/src/bootstrap.sh", "docker"]