Expose port via Dockerfile

Dockerfile

@@ -6,4 +6,6 @@ COPY Gemfile Gemfile.lock /app/
 RUN apk add --no-cache build-base && bundle install && apk del build-base
 COPY . /app
 
+EXPOSE 80
+
 ENTRYPOINT ["/usr/local/bin/bundle", "exec", "rackup", "-E", "production", "-p", "80"]