unix: automatically remove container created by mkall.sh

When GOOS=linux, mkall.sh creates a container to run linux/mkall.go.  It does not
remove this container before exiting.  Therefore each time you run mkall.sh on Linux
you end up with a new stopped container that must be manually deleted with docker rm.
This is cumbersome as it may take several runs of mkall.sh to develop and test a patch
for golang/x/sys/unix resulting in multiple containers that require manual removal.
It's also a little counterintuitive as the user doesn't invoke the docker command directly
so it's not obvious that manual cleanup is required after mkall.sh completes.  The
leftover containers aren't even that useful for debugging as they have an entrypoint
set.

Change-Id: I92a94ae53078f0095d3ddf1d95c4879f1bc1cb2a
Reviewed-on: https://go-review.googlesource.com/c/sys/+/513335
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>