don't do an extra yarn install in docker land
don't do an extra yarn install in docker land

this makes it so building the web container succeeds without having to
tweak the Dockerfile. i.e. no more:

  Error: ENOENT: no such file or directory, scandir '/usr/src/app/node_modules/node-sass/vendor'

i'm still not sure why the extra yarn install causes that, but it does :shrug:

test plan:
`docker-compose build web` should succeed

Change-Id: I5120dd0ea6168d7d373364bccd02bfa990df54d5
Reviewed-on: https://gerrit.instructure.com/111953
Tested-by: Jenkins
Reviewed-by: Landon Wilkins <e219b0c23c4256074cdfc7a23138e4f17200b08f@instructure.com>
Product-Review: Landon Wilkins <e219b0c23c4256074cdfc7a23138e4f17200b08f@instructure.com>
QA-Review: Landon Wilkins <e219b0c23c4256074cdfc7a23138e4f17200b08f@instructure.com>

sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,venturehive/canvas-lms,instructure/canvas-lms,sfu/canvas-lms