Bug 1443437 - Pre-build font cache for Firefox i386 binaries on x86_64 systems. r=jmaher

In automation for both Linux and Linux64 the same docker image is used,
which is based on Ubuntu 16.04 64bit. To run the 32bit Firefox binary
the necessary libraries for the i386 architecture get installed.

What's missing is the creation of the font cache for i386 binaries. As
such it will be created at the first start of the appropriate application.
It means there is an additional ~30s delay before Firefox opens, which
causes several test failures especially for wdspec tests.

To prevent this delay the i386 version of fontconfig will be installed
and apt automatically creates the font cache for the i386 architecture
by leaving the x86_64 cache in-place.

So the cache creation will happen one time  when the docker image is getting
build, and not for each and every test job, which saves us 30s for each.

MozReview-Commit-ID: 58xdOI4xYkP

--HG--
extra : rebase_source : f246fc47caa1acc8657c4963c9450c1ca7653fec

taskcluster/docker/recipes/ubuntu1604-test-system-setup.sh

@@ -167,6 +167,10 @@ apt-get -q -y -f install \
     libdbus-glib-1-2:i386 \
     openjdk-8-jdk:i386
 
+# use fc-cache:i386 to pre-build the font cache for i386 binaries
+apt-get -q -y -f install \
+    fontconfig:i386 \
+
 # revert the list of repos
 cp sources.list.orig /etc/apt/sources.list
 apt-get update