Use smaller docker image
Use smaller docker image

chefes/releng-base docker image is more than 8GB uncompressed.
There is far more in that image than we need for testing omnibus-software.
This wastes time downloading the image and
disk space. We can use Ubuntu:18.04 instead which is about 56MB uncompressed.

Signed-off-by: Jeremiah Snapp <a9df78b4b5c00745f26b0821b2cc57336a474862@chef.io>

chef/omnibus-software,chef/omnibus-software,chef/omnibus-software,chef/omnibus-software,chef/omnibus-software,chef/omnibus-software