add slack alert for failed docker-dev cron jobs
add slack alert for failed docker-dev cron jobs

flag = none
closes: DE-294

Change-Id: Ib88185e9c8207098f215e4a0e3f442f9d273dbd7
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/249189
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Kyle Rosenbaum <08c6582752644f4eca90309d6d9805aeb3128ceb@instructure.com>
Reviewed-by: Andrea Cirulli <b4e55ef5ee4bae8ad31f7c81009bfc0af1714a1f@instructure.com>
QA-Review: James Butters <4c5a489c9bb72d41f20b3b22aaf4c9b864e32f20@instructure.com>
Product-Review: James Butters <4c5a489c9bb72d41f20b3b22aaf4c9b864e32f20@instructure.com>

instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms