WIP: Customized docker image, part 1/2

This is part 1 of a 2-part MR to add and demonstrate building a custom Docker image for Weston to use in CI testing.

Using a custom image will avoid installing the same distribution packages over and over again on each CI run, making the runs faster.

This part needs to be merged first, which triggers an image build in the upstream master branch if all goes well. Once the image is built, part 2 !64 (closed) needs to drop the WIP commit and re-run CI, after which that one will be ready to merge.

Everyone who wants to change the image build in the future will need to follow a similar 2-part MR process. Instructions are in the Dockerfile.