rake release_docker + smaller image builds

Dockerfile

@@ -1,10 +1,9 @@
 FROM ruby:alpine
 RUN mkdir /share
-RUN apk add --update \
-  build-base \
-  libxml2-dev \
-  libffi-dev
-RUN gem install inspec --no-document
+COPY inspec.gem /
+RUN apk add --update build-base libxml2-dev libffi-dev && \
+    gem install /inspec.gem --no-document && \
+    apk del build-base
 ENTRYPOINT ["inspec"]
 CMD ["help"]
 VOLUME ["/share"]

Rakefile

@@ -146,3 +146,14 @@ task :bump_version, [:version] do |_, args|
   inspec_version(v)
   Rake::Task['changelog'].invoke
 end
+
+desc 'Release a new docker image'
+task :release_docker do
+  version = Inspec::VERSION
+  cmd = "rm *.gem; gem build *gemspec && "\
+        "mv *.gem inspec.gem && "\
+        "docker build -t chef/inspec:#{version} . && "\
+        "docker push chef/inspec:#{version}"
+  puts "--> #{cmd}"
+  sh('sh', '-c', cmd)
+end