Add a libplacebo docker image

Some notes / open questions:

• Ubuntu cosmic has the vulkan validation layers, older versions do not. So I decided to add a new cosmic base image
• We'll actually need to figure out how to get a GPU (or swrast) into the VM in order to be able to do useful testing (apart from testing whether it builds)
• I don't know how your tagging system works, so I just put the FROM line without any tags. Please either amend this yourself or explain it in the README
• We build shaderc "by hand". Not strictly necessary, but I think I want an environment in which I can test this
• We do a bare git clone in the docker image, of the current git master at the point in time the docker image is built. I'm not sure what your policy here is, but should we pin specifics commits to make sure the docker image is reproducible?