Add ubuntu focal docker images

[sloretz]

I think this might be needed for http://build.ros.org/view/Nsrc_uF/job/Nbin_ufv8_uFv8__catkin__ubuntu_focal_arm64__binary/1/console .

00:00:14.077 # BEGIN SECTION: Check docker status
00:00:14.077 + echo Testing trivial docker invocation...
00:00:14.077 Testing trivial docker invocation...
00:00:14.077 + docker run --rm osrf/ubuntu_arm64:focal true
00:00:14.130 Unable to find image 'osrf/ubuntu_arm64:focal' locally
00:00:14.999 docker: Error response from daemon: manifest for osrf/ubuntu_arm64:focal not found: manifest unknown: manifest unknown.

[tfoote]

Yeah, we'll need this added then run.

[sloretz]

@tfoote I got a failure to build the osrf/ubuntu_i386:focal image. Before I investigate, does anything need an i386 image for focal, or can I ignore and remove that from the list?

[tfoote]

We can remove it from the list. It would be nice to have but if it's failing i386 is now rarely used so we shouldn't spend resources trying to make it work.