clarify dockerfile linters message
clarify dockerfile linters message

Change-Id: Ib2c5627017bdcbc5e75890e7287128a7b628f38d
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/281102
Reviewed-by: Ryan Norton <65299b45178f94900ff304765e8fbf07bd13b331@instructure.com>
Reviewed-by: Andrea Cirulli <b4e55ef5ee4bae8ad31f7c81009bfc0af1714a1f@instructure.com>
Product-Review: Ryan Norton <65299b45178f94900ff304765e8fbf07bd13b331@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Aaron Ogata <11ccf682c06d1508642f943a3962a1c3d00b8e44@instructure.com>

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