Update getting_docker.md
Update getting_docker.md

Test plan:
 - Specs Pass

closes CNVS-50088
closes GH-1633
flag = none

Change-Id: Ib53106edf48d09e93eceacdc2ffa326579dbbb58
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/246747
Reviewed-by: Aaron Ogata <11ccf682c06d1508642f943a3962a1c3d00b8e44@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
QA-Review: Alex Slaughter <11669a22a05ead38ed9badafd55ef4c5386724b6@instructure.com>
Product-Review: Alex Slaughter <11669a22a05ead38ed9badafd55ef4c5386724b6@instructure.com>

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