Move common canvas/pathkit dockerfiles to more intuitive location

Also rename emsdk-base -> emsdk-release so it is consistent with
the container name.

Bug: skia:
NOTRY: true
Change-Id: Idec19bdeb64a64120d0890b2987c2dce0a57535a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206909
Reviewed-by: Stephan Altmueller <stephana@google.com>
15 files changed