[TSK]: Speed up reuse action with Docker

[dkfellows]

What do you want us to do?

Currently, our license checking is a rather slow workflow because the reuse action is itself rather slow, which in turn is mostly due to the cost of setting up a Python virtual environment and installing reuse-tool and its dependencies into it. We should find some way to accelerate this.

The obvious option is to build things into a Docker image, as pulling a Docker image from the GHCR is quite fast.

[dkfellows]

Note that there's a number of other actions (candidates: those that in turn call the apt-get-install action) where this treatment might be valid. Developing a more general scheme for publishing these images may be advisable.

[dkfellows]

Example of a docker-based action: https://github.com/mattnotmitt/doxygen-action