• Fixes Many parallel calls to docker exec make Docker unresponsive #45595
• Workaround for Concurrent task execs take significantly longer than sequentially containerd/containerd#8557

- What I did
It turns out that the unnecessary serialization removed in b752462 happened to work around a bug in containerd. When many exec processes are started concurrently in the same containerd task, it takes seconds to minutes for them all to start. Add the workaround back in, only deliberately this time.

- How I did it
✨ mutex ✨

- How to verify it

# docker run --rm -d --name pause1 alpine sleep infinity
# time sh -c 'myjobs=""; for i in $(seq 1 50); do docker exec pause1 true & myjobs="$myjobs $!"; done; for pid in $myjobs; do wait $pid; done'

Takes ~3 seconds on my machine, compared to around a minute without the workaround.

- Description for the changelog

• Work around an issue in containerd which causes execs to take an excessive amount of time to start

- A picture of a cute animal (not mandatory but encouraged)