* Adds a shell script with functions to install multi-architecture docker
support, as well as build, deploy, and test the shellcheck docker images for
the same set of architectures for which binaries were already built and
deployed as tarballs.
* Hooks up the multi-architecture docker build, deploy, and test to the existing
Travis CI/CD pipeline. It is organized as a separate stage which only runs if
all previous steps in the already existing test stage succeed.
Artur Klauser