Install ffmpeg into taskcluster image

This is required for codec support in Firefox

.taskcluster.yml

diff --git a/.taskcluster.yml b/.taskcluster.yml
index d9e5581..da39156 100644
--- a/.taskcluster.yml
+++ b/.taskcluster.yml
@@ -57,7 +57,7 @@
             owner: ${owner}
             source: ${event.repository.clone_url}
           payload:
-            image: webplatformtests/wpt:0.56
+            image: webplatformtests/wpt:0.57
             maxRunTime: 7200
             artifacts:
               public/results:

tools/ci/tc/tasks/test.yml

diff --git a/tools/ci/tc/tasks/test.yml b/tools/ci/tc/tasks/test.yml
index 11042a4..a3d70eb 100644
--- a/tools/ci/tc/tasks/test.yml
+++ b/tools/ci/tc/tasks/test.yml
@@ -4,7 +4,7 @@
     workerType: ci
     schedulerId: taskcluster-github
     deadline: "24 hours"
-    image: webplatformtests/wpt:0.56
+    image: webplatformtests/wpt:0.57
     maxRunTime: 7200
     artifacts:
       public/results:

tools/docker/Dockerfile

diff --git a/tools/docker/Dockerfile b/tools/docker/Dockerfile
index a916255..736f153 100644
--- a/tools/docker/Dockerfile
+++ b/tools/docker/Dockerfile
@@ -13,6 +13,7 @@
     curl \
     dbus-x11 \
     earlyoom \
+    ffmpeg \
     fonts-noto \
     fluxbox \
     gdebi \