panic on program exit

[mischief]

when the program run by dockerize exits, dockerize panics with a deadlock.

fatal error: all goroutines are asleep - deadlock!

goroutine 16 [semacquire, 35 minutes]:
sync.runtime_Semacquire(0xc208034298)
/usr/local/go/src/pkg/runtime/sema.goc:199 +0x30
sync.(*WaitGroup).Wait(0x651980)
/usr/local/go/src/pkg/sync/waitgroup.go:129 +0x14b
main.main()
/home/jwilder/go/src/github.com/jwilder/dockerize/dockerize.go:93 +0x983

goroutine 19 [finalizer wait, 37 minutes]:
runtime.park(0x415720, 0x6511d0, 0x64fd29)
/usr/local/go/src/pkg/runtime/proc.c:1369 +0x89
runtime.parkunlock(0x6511d0, 0x64fd29)
/usr/local/go/src/pkg/runtime/proc.c:1385 +0x3b
runfinq()
/usr/local/go/src/pkg/runtime/mgc0.c:2644 +0xcf
runtime.goexit()
/usr/local/go/src/pkg/runtime/proc.c:1445