unix: update Dockerfile to Go 1.12 and Linux 5.0

Also enable KexecFileLoad on linux/{arm64,riscv64} now that they have
SYS_KEXEC_FILE_LOAD.

Change-Id: I1c52d09a1a617794f2df697e4966c4c24a166611
Reviewed-on: https://go-review.googlesource.com/c/sys/+/164997
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_linux_arm64.go

@@ -208,3 +208,16 @@ func Poll(fds []PollFd, timeout int) (n int, err error) {
 	}
 	return ppoll(&fds[0], len(fds), ts, nil)
 }
+
+//sys	kexecFileLoad(kernelFd int, initrdFd int, cmdlineLen int, cmdline string, flags int) (err error)
+
+func KexecFileLoad(kernelFd int, initrdFd int, cmdline string, flags int) error {
+	cmdlineLen := len(cmdline)
+	if cmdlineLen > 0 {
+		// Account for the additional NULL byte added by
+		// BytePtrFromString in kexecFileLoad. The kexec_file_load
+		// syscall expects a NULL-terminated string.
+		cmdlineLen++
+	}
+	return kexecFileLoad(kernelFd, initrdFd, cmdlineLen, cmdline, flags)
+}