Bug 1501228 [wpt PR 13676] - Update docker image to Ubuntu 18.04, a=testonly

Automatic update from web-platform-testsUpdate docker image to Ubuntu 18.04 (#13676)

This is the latest LTS release, so it makes sense to test on it. Also it is a requirement
to get servo builds running in the docker image since 16.04 has a too-old gstreamer.
--

wpt-commits: fbc56f388ac5d3a9cfcdfc161917ba0cfa2280f5
wpt-pr: 13676

testing/web-platform/tests/.taskcluster.yml

@@ -61,7 +61,7 @@ tasks:
               owner: ${event.pusher.email}
               source: ${event.repository.url}
             payload:
-              image: jugglinmike/web-platform-tests:0.21
+              image: harjgam/web-platform-tests:0.22
               maxRunTime: 7200
               artifacts:
                 public/results:

testing/web-platform/tests/tools/docker/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 
 # No interactive frontend during docker build
 ENV DEBIAN_FRONTEND=noninteractive \