Create /etc/resolver when setting up dinghy-proxy

Seems that directory doesn't always exist on MacOS

flag=none

test plan:
- Check whether /etc/resolver exists; if it does (and is empty), remove it
  - If it's not empty, back up its contents
- Run docker_dev_setup.sh
- Confirm that it succeeds with the Dinghy setup


Change-Id: I2958a798907c2e450645ee251348ef79e499f657
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/269085
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Product-Review: Jon Scheiding <jon.scheiding@instructure.com>
Reviewed-by: Nate Armstrong <narmstrong@instructure.com>
QA-Review: Nate Armstrong <narmstrong@instructure.com>

script/common/utils/dinghy_setup.sh

@@ -68,6 +68,8 @@ function setup_dinghy_proxy {
     --name http-proxy \
     codekitchen/dinghy-http-proxy
 
+    sudo mkdir -p /etc/resolver
+
     echo 'nameserver 127.0.0.1' | sudo tee /etc/resolver/docker > /dev/null
     echo 'port 19322' | sudo tee -a /etc/resolver/docker > /dev/null
   fi