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This is to do with how docker signals the container.

When you run docker stop ..., some things will happen:

1. docker sends a SIGTERM to the main process of the container. The process is able to mask/ignore a SIGTERM, and if it does so (or handles it without terminating) "nothing" will happen.
2. After a timeout (default 10 seconds), docker sends a SIGKILL to the main process. This signal cannot be masked by a process, and thus it dies immediately with no opportunity for executing a shutdown prodedure.

Ideally, processes run within docker will respond to the SIGTERM in a timely fashion, taking care of any housekeeping before terminating.

If you know that the process either doesn't have any housekeeping to perform (e.g: sleep), or will not respond properly to SIGTERM, you can specify a shorter (or longer) timeout with the -t flag:

-t, --time=10

    Seconds to wait for stop before killing it

For example, in your case, you may like to run docker stop -t 0 ${CONTAINER}.

The reason that this signal behaviour is different is due to the sleep running with PID = 1.

Typically (e.g: running with PID != 1), any signal that the process doesn't explicitly deal with leads to the process being terminated - try sending a sleep a SIGUSR1.

However, when running with PID = 1, unhandled signals are ignored.

You can send a signal to the docker container using docker tools, for example:

docker kill -s TERM kill-sleep

As we can see, this doesn't have the desired effect, whereas this does:

docker kill -s KILL kill-sleep