change ci-docker.sh to use sh instead of bash (#2068)

diff --git a/docker/ci-docker.sh b/docker/ci-docker.sh
index d891976..d6a2678 100755
--- a/docker/ci-docker.sh
+++ b/docker/ci-docker.sh
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 set -o errexit
 
 realpath() {