Metadata for container execution

At the ESAP tech meeting of 2022-05-02, @dmorris demonstrated the metadata system that he proposes to use to describe software which can be executed through ESAP. This enables the user to e.g. describe the details of a container to be executed.

As a proof of concept, demonstrate that this metadata provides sufficient information to launch a container containing a graphical application (ie, as described in &9).