remove dead skip-docker-build flag
remove dead skip-docker-build flag

refs DE-458

This flag hasn’t worked for a long time, and caching has made this stage fast enough now that it isn’t needed.

Change-Id: I138c5716c35bcde09a28269f27dafe772a01fcb4
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/260259
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Kyle Rosenbaum <08c6582752644f4eca90309d6d9805aeb3128ceb@instructure.com>
QA-Review: Aaron Ogata <11ccf682c06d1508642f943a3962a1c3d00b8e44@instructure.com>
Product-Review: Aaron Ogata <11ccf682c06d1508642f943a3962a1c3d00b8e44@instructure.com>

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