In the case you're a Rails developer working with Docker, migrating your application to Rails 4.2 introduced the issue that when starting the application, you cannot access it anymore.
Having a closer look to the logs shows us why:
=> Booting Puma => Rails 4.2.0 application starting in development on http://localhost:3000 => Run `rails server -h` for more startup options => Ctrl-C to shutdown server Puma 2.11.0 starting... * Min threads: 0, max threads: 16 * Environment: development * Listening on tcp://localhost:3000
As you can see the Rails application is listening to localhost instead of 0.0.0.0.
The Rack default option of the binding parameter has been updated as described in this Github issue.
Fixing the issue with Fig
With Fig, updating the
fig.yml file solves the issue:
web: build: . command: bundle exec bin/rails server -b 0.0.0.0 ...
-b 0.0.0.0 fixes the issue.