Update dockerfile with locale

.ci/Dockerfile

@@ -0,0 +1,28 @@
+ARG ELASTIC_STACK_VERSION
+ARG DISTRIBUTION_SUFFIX
+FROM docker.elastic.co/logstash/logstash${DISTRIBUTION_SUFFIX}:${ELASTIC_STACK_VERSION}
+RUN sudo localedef -i en_US -f UTF-8 en_US.UTF-8
+RUN locale
+RUN locale -a
+RUN localectl set-locale LANG=en_US.UTF-8
+USER logstash
+
+COPY --chown=logstash:logstash Gemfile /usr/share/plugins/plugin/Gemfile
+COPY --chown=logstash:logstash *.gemspec VERSION* version* /usr/share/plugins/plugin/
+RUN cp /usr/share/logstash/logstash-core/versions-gem-copy.yml /usr/share/logstash/versions.yml
+# NOTE: since 8.0 JDK is bundled as part of the LS distribution under $LS_HOME/jdk
+ENV PATH="${PATH}:/usr/share/logstash/vendor/jruby/bin:/usr/share/logstash/jdk/bin"
+ENV LOGSTASH_SOURCE="1"
+ENV ELASTIC_STACK_VERSION=$ELASTIC_STACK_VERSION
+# DISTRIBUTION="default" (by default) or "oss"
+ARG DISTRIBUTION
+ENV DISTRIBUTION=$DISTRIBUTION
+# INTEGRATION="true" while integration testing (false-y by default)
+ARG INTEGRATION
+ENV INTEGRATION=$INTEGRATION
+RUN gem install bundler -v '< 2'
+WORKDIR /usr/share/plugins/plugin
+RUN bundle install --with test ci
+COPY --chown=logstash:logstash . /usr/share/plugins/plugin
+RUN bundle exec rake vendor
+RUN .ci/setup.sh