Timeout during `deploy:compile_assets`

### Configuration

https://gist.github.com/DalfDev/eddaacb4f625c7f63f9c8e20f5dc26c9

### Expected behavior

It should compile assets and deploy after and then terminate compilation processes.

### Actual behavior

Most of the compilation of our assets timeout. Plus, the processes are staying on our server.

On this screenshot (taken on 2020-09-10), you can see two compilations from the day before (2020-09-09):
https://image.noelshack.com/fichiers/2020/37/5/1599831999-screenshot-2020-09-10-at-09-03-55.png

Here an example of a `deploy:compile_assets` we had to interrupt after a while:
https://image.noelshack.com/fichiers/2020/37/5/1599831998-screenshot-2020-09-11-at-15-27-57.png

This seems to happen when processes are not killed. After a reboot, everything works fine **the first time** but killing the remaining processes manually is not enough to fix the problem.

This problem happened since we upgraded our rails version from `5.2.0` to `6.0.3`.

### System configuration

https://gist.github.com/DalfDev/153324537f86924edf504f443e0100b5

Start at the deploy:compile_assets entry point and review the configuration and system details in the linked gists. Compare the behavior before and after the Rails 5.2.0 to 6.0.3 upgrade, using the screenshots as symptom evidence. Done means asset compilation completes, deployment proceeds, and compilation processes terminate without manual cleanup.

Written by the indexing model from the issue text.