Add docker based build to simplify process

Dockerfile

@@ -0,0 +1,9 @@
+FROM jruby:latest
+
+WORKDIR /usr/src/plugin
+
+COPY Gemfile *.gemspec ./
+
+RUN bundle install
+
+COPY . .

build.sh

@@ -0,0 +1,4 @@
+#!/bin/bash
+docker build -t logstash-jruby .
+docker run -v $(pwd):/usr/src/plugin logstash-jruby \
+  gem build logstash-output-salesforce.gemspec

logstash-output-salesforce.gemspec

@@ -10,7 +10,7 @@ Gem::Specification.new do |s|
   s.require_paths = ["lib"]
 
   # Files
-  s.files = `git ls-files`.split($\)
+  s.files = `find .  -type f -not -path '*/\.*'`.split($\)
 
   # Tests
   s.test_files = s.files.grep(%r{^(test|spec|features)/})