How to conect to a dockerized Sinatra app - ruby

Question

I'm trying to dockerize a simple Sinatra application, but when Docker is started I can't access the API through the exposed port 4567.
This is my Dockerfile:
FROM ruby:2.7
RUN apt-get update -qq && apt-get install -y build-essential
RUN mkdir /app
WORKDIR /app
COPY Gemfile /app/Gemfile
COPY Gemfile.lock /app/Gemfile.lock
RUN bundle install
COPY . /app
EXPOSE 4567
CMD [ "bundle", "exec", "ruby", "main.rb", "-p", "4567" ]
then:
docker build -t my-application .
docker run -p 4567:4567 my-application
when I visit localhost:4567:
enter image description here
but if I access the container and make a curl request, it works:
docker exec -it bdcbfcaa57aa bash
enter image description here
I don't see what I'm doing wrong.

Answer 1

It is most likely that your Sinatra application listens on 127.0.0.1. You need to bind it to 0.0.0.0 instead.
In classic Sinatra, it's done with:
require 'sinatra'
set :port, 4567
set :bind, '0.0.0.0'
# ... rest of the app
I am not sure what is in your main.rb, but you would probably want to add support for --host 0.0.0.0 or --bind 0.0.0.0.
Here is a fully functional example:
Dockerfile:
FROM dannyben/alpine-ruby
WORKDIR /app
COPY . .
RUN gem install puma sinatra
EXPOSE 3000
CMD ruby server.rb
server.rb:
require 'sinatra'
set :port, 3000
set :bind, '0.0.0.0'
get '/' do
"we are the champions"
end
Then run:
$ docker build -t temp .
$ docker run --rm -it -p 3000:3000 temp