[Skiboot] [PATCH] travis/ci: rework Dockerfiles to produce build artifacts

[Stewart Smith]

Stewart Smith <stewart at linux.ibm.com> writes:
> ubuntu-latest was also missing clang, as ubuntu-latest is closer to
> ubuntu 18.04 than 16.04
>
> Signed-off-by: Stewart Smith <stewart at linux.ibm.com>
> ---

Merged to master as of d513609b4f48d188dcd2e9ba8af831a92464d15c, as this
makes a bunch of my CI and testing things a bit saner. Why I didn't
originally do it this way is anybody's guess.

-- 
Stewart Smith
OPAL Architect, IBM.