client/android: fix container detection for docker in systemd cgroup

It seems that on other (more modern?) configurations, the cgroup for
processes in docker is /system.slice/docker.service instead of /docker.

I have not been able to reproduce such a configuration. But until we have
a better way to detect whether a process is in docker, it's probably ok
to just add the new path to validate the detection, which this change
does.

Fixes #973

Change-Id: I9d52bcd79a906fdc1f8310b2c8063930cf9807fb

clients/android/build-in-docker.go

@@ -152,7 +152,8 @@ func inDocker() bool {
 		if len(fields) != 3 {
 			log.Fatal(`unexpected line in "/proc/self/cgroup"`)
 		}
-		if !strings.HasPrefix(fields[2], "/docker/") {
+		if !(strings.HasPrefix(fields[2], "/docker/") ||
+			strings.HasPrefix(fields[2], "/system.slice/docker.service")) {
 			return false
 		}
 	}