unix: mkall.sh: fail if docker build failed

When docker build fails, docker run (in my case podman) tries to find
the generate:linux image from various mirrors, which is very confusing.
We should error out if docker build fails.

Change-Id: I4fd78e9fa339e03029b1bf003b3239ca23a7ed1b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/706916
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Kirill Kolyshkin <kolyshkin@gmail.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Carlos Amedee <carlos@golang.org>