Update docker images

[GeoffClements]

Noting #85, the docker image for armv7-unknown-linux-gnueabihf is based on ubuntu:14.04 which is okay providing you don't need to link to more modern libraries.

In my case I use the gstreamer crate which requires gstreamer-1.0 which is not available in ubuntu:14.04.

Please can you update to ubuntu:18.04.

[Dylan-DPC-zz]

Hi thanks for reporting this.

However do check this comment that explains why we haven't upgraded yet

[GeoffClements]

@Dylan-DPC Ok, thanks for the link.

That's means I will probably have to roll my own docker image based on Ubuntu or Debian late enough to support gstreamer1.0. I am aiming to run on RPi2/3 with Tiny Core Linux which currently uses glibc 2.26 so it looks possible.

I have already tried a docker image based on Ubuntu:18.04 for the armhf architecture. In this case cross is not needed because I planned to run the arm versions of cargo and rustc through binfmt services on the host. It didn't work, I got many compile errors.

[Alexhuszagh]

FYI: this has been done since #591.