Require Fuchsia arm64 test swarming bots to run in docker containers.

This helps to standardize the environment for the Fuchsia VM.
In particular, it prevents tests from running on build41-b4 which
is not set up to run Fuchsia arm64 KVM tests.

Bug: 1000901
Change-Id: I0030cb7dc807829404590f23cdb4a8595f93b1a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1918312
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Stephen Roe <steveroe@google.com>
Cr-Commit-Position: refs/heads/master@{#715932}