Start Guard instead of sleeping in dev environment

[top4ek]

Ticket

https://community.openproject.org/projects/openproject/work_packages/68729/activity

What's going on

Allow developers to start Guard inside backend-test container instead of sleeping. This allows to run tests automatically on source files change while existing testing method via docker exec still can be used.

Guard can be started by setting START_GUARD environment variable to "1". Otherwise it's behavior stayed the same. (Do we need that?)

Also, we can use docker attach and stop code with binding.break(byebug as temporary we using it) because of tty and stdin options enabled in compose files.