I made a demo that tries to reproduce the issue i face. You could find it here: https://github.com/denisj/app-not-booting.
I can't find the reason why the app is not responding. The logs for the app container look like that:
Attaching to test-ruby-3-rails-6_app_1
app_1 | Puma starting in single mode...
app_1 | * Puma version: 5.1.1 (ruby 3.0.0-p0) ("At Your Service")
app_1 | * Min threads: 5
app_1 | * Max threads: 5
app_1 | * Environment: development
app_1 | * PID: 1
app_1 | * Listening on http://0.0.0.0:3000
app_1 | Use Ctrl-C to stop
and for the web container:
web_1 | 172.28.0.1 - - [02/Feb/2021:16:28:08 +0000] "GET / HTTP/1.1" 499 0 "-" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.66 Safari/537.36"
And few seconds later, i'm getting: 504 Gateway Time-out.
The only thing I've found is that it seems to have something to do with these 3 gems:
gem 'password_strength', '~> 1.1'
gem 'slack-ruby-client', '~> 0.15'
gem 'asset_sync', '~> 2.12'
When I disable one, the app is working (I mean I hope so).