Fix HOME directory in Docker containers built under cloudbuild.

The line:
    USER skia

also sets the $HOME environment variable to /home/skia unless
useradd is given another directory to use as the user's home
directory. So HOME was set to /home/skia, but that directory
doesn't exist in subsequent containers.

Bug: chromuim:993053
Change-Id: I3525d7cf11a1c1e5559cf9216c3bf6071a236949
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/244116
Reviewed-by: Ravi Mistry <rmistry@google.com>
Reviewed-by: Robbie Iannucci <iannucci@google.com>
Commit-Queue: Joe Gregorio <jcgregorio@google.com>
3 files changed