Bug 1632329 [wpt PR 23190] - Add python3 and python3-pip to the docker image, a=testonly

Automatic update from web-platform-tests
Add python3 and python3-pip to the docker image (#23190)

See
https://github.com/web-platform-tests/wpt/pull/23182#issuecomment-618054801

Update README too

--

wpt-commits: cbb6f674bad9b7f58845d4b5fd5b232effea5f4d
wpt-pr: 23190

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

@@ -56,7 +56,7 @@ tasks:
             owner: ${owner}
             source: ${event.repository.clone_url}
           payload:
-            image: hexcles/web-platform-tests:0.35
+            image: webplatformtests/wpt:0.36
             maxRunTime: 7200
             artifacts:
               public/results:

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

@@ -25,6 +25,8 @@ RUN apt-get -qqy update \
     pulseaudio \
     python \
     python-pip \
+    python3 \
+    python3-pip \
     qemu-kvm \
     tzdata \
     sudo \

testing/web-platform/tests/tools/docker/README.md

@@ -1,11 +1,17 @@
 This docker images is used for testing Chrome, Firefox, WebKitGTK and running
 other tasks on Taskcluster. When any of the files in this directory change, the
-images must be updated as well. To do this, assuming you have docker installed:
+images must be updated as well. Doing this requires you be part of the
+'webplatformtests' organization on Docker Hub; ping @Hexcles or @stephenmcgruer
+if you are not a member.
+
+In this directory, run the following, where `<tag>` is of the form
+`webplatformtests/wpt:{current-version + 0.01}`:
 
-In this directory, run
 ```sh
 # --pull forces Docker to get the newest base image.
 docker build --pull -t <tag> .
 docker push <tag>
 ```
 
+Then update `.taskcluster.yml` in the top-level directory to point to the new
+image you have uploaded.