Build fails for crates that uses ring 1.3.5

[TotalKrill]

ring issue

It is fixable by changing the docker image to use a more recent version of Ubuntu,

should the Ubuntu version be upgraded, or could the ubuntu version be settable somehow?

[Alexhuszagh]

Relevant issues and PRs include #557 and #591, which should upgrade the Ubuntu to a more version. Unfortunately, due to some packages and especially images with Linux headers, allowing an arbitrary Ubuntu version is much more difficult than it sounds.

[Alexhuszagh]

No longer replicates as of the latest commit for ring (briansmith/ring@0f3bf00):

$ git clone https://github.com/briansmith/ring
$ cd ring
$ cross build --target aarch64-unknown-linux-gnu
$ cross test --target aarch64-unknown-linux-gnu

All pass. We're also going to be updating the images in #591 for the v0.3.0 release.