improve docker-dev-changes.sh
improve docker-dev-changes.sh

the current grep is including files we don't actually want, this
improvement will help narrow down to the file changes we actually
want to trigger the docker-dev build.

flag = none

Change-Id: I2d689ba9d00dd0955e4ea333f5f5fbd92c2f1f28
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/248817
Reviewed-by: Aaron Ogata <11ccf682c06d1508642f943a3962a1c3d00b8e44@instructure.com>
Reviewed-by: Ryan Norton <65299b45178f94900ff304765e8fbf07bd13b331@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: James Butters <4c5a489c9bb72d41f20b3b22aaf4c9b864e32f20@instructure.com>
Product-Review: James Butters <4c5a489c9bb72d41f20b3b22aaf4c9b864e32f20@instructure.com>

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