Use fully-qualified container image references

[JamieMagee]

We use a lot of short container references like ubuntu:20.04 or containerbase/base. But these can sometimes be confusing, especially as we are also pushing images to the GitHub container registry ghcr.io. And they are also not standardized across container runtimes. For example, running podman pull ubuntu:20.04 will result in the error Error: short-name "ubuntu:20.04" did not resolve to an alias.

Instead we should use fully-qualified references, like docker.io/library/ubuntu:20.04 or docker.io/containerbase/base.