Bug: Reaper is force-removed at interpreter exit since 4.15.0, session containers are never reaped #1140

Describe the bug

Since 4.15.0, containers started by a process are left running after that process exits normally. Ryuk never reaps them.

#1017 registered Reaper.delete_instance with atexit to close the Ryuk socket before interpreter shutdown. delete_instance() also calls Reaper._container.stop(), which is remove(force=True), so the Ryuk container gets SIGKILL at the same moment the socket closes:

https://github.com/testcontainers/testcontainers-python/blob/6146aa1/src/testcontainers/core/container.py#L544

Ryuk only reaps a session RYUK_RECONNECTION_TIMEOUT (10s by default) after the last connection drops. Killed at exit, it never gets there. Every container of a session that ends normally without stopping its containers itself stays behind. This is the common case Ryuk exists for, e.g. a session-scoped pytest fixture that relies on Ryuk for cleanup.

docker events for the Ryuk container at interpreter exit:

start   <ryuk id>
kill    <ryuk id> sig=9
destroy <ryuk id>

On 4.14.2 the same script leaves nothing behind. With the process ending through os._exit(0), which skips atexit, 4.15.0 works too, and the Ryuk log shows the normal path:

Client disconnected: 172.17.0.1:49870
Timeout waiting for connection
Removed 1 container(s), 0 network(s), 0 volume(s) 0 image(s)

Expected: the process exits, Ryuk removes the session's containers about 10s later.
Actual: Ryuk is force-removed at exit, the session's containers keep running.

To Reproduce

python -c "
from testcontainers.core.container import DockerContainer
c = DockerContainer('alpine:3.20').with_command('sleep infinity').start()
print(c.get_wrapped_container().short_id)
"
sleep 15
docker ps --filter label=org.testcontainers.lang=python
# 4.14.2: empty. 4.15.0 and main: the alpine container is still Up.

Workarounds: pin testcontainers<4.15, or drop the handler after the reaper is created with atexit.unregister(Reaper.delete_instance).

Closing only the socket at exit keeps the fix from #1017 (no ResourceWarning) and lets Ryuk do its job. #1125 hits a 409 from this same atexit call when Ryuk is already being removed. Fixing this means the atexit path no longer runs into it. I'll open a PR with a fix and a regression test.

Runtime environment

• testcontainers 4.15.0, reproduced on main at 6146aa1
• Python 3.12.12
• macOS with Colima (Ubuntu 24.04.4 VM, Docker Engine 29.2.1)