[infra] Use POSIX paths for Docker containers

This makes the Housekeeper-PerCommit-InfraTests_Win task less broken.

Change-Id: Icd82b172dede83b69ed415fb03c3c47b54b5df5d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/261997
Auto-Submit: Ben Wagner aka dogben <benjaminwagner@google.com>
Reviewed-by: Ravi Mistry <rmistry@google.com>
Commit-Queue: Ravi Mistry <rmistry@google.com>
2 files changed