merge webpack-cache into single prefix
merge webpack-cache into single prefix

refs DE-407

Now that build arguments are included in the cache ID, we don’t need to have a separate prefix anymore for each type of image.

[change-merged]
[build-registry-path=jenkins/canvas-lms/de-407]

Change-Id: I940bf09b856dc57a9256633fe3a96fac98219c6c
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/254625
Reviewed-by: Andrea Cirulli <b4e55ef5ee4bae8ad31f7c81009bfc0af1714a1f@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@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,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms