lock docker file to ruby 2.5.3

lib/natra/generators/app/templates/Dockerfile

@@ -1,13 +1,25 @@
-FROM ruby AS base
+FROM ruby:2.5.3 AS base
+
 WORKDIR /usr/src/app
+
 RUN gem install bundler
+
 COPY Gemfile /usr/src/app/
+
 FROM base AS development
+
 RUN bundle install -j5 --without staging production
+
 COPY . /usr/src/app
+
 FROM base AS release
+
 RUN bundle install -j5 --without development test
+
 RUN chmod 755 ./usr/src/app
+
 RUN chmod 755 ./usr/src/app/bin/setup
+
 COPY . /usr/src/app
+
 CMD puma