Locking gitlab CI to Hugo version 0.62.2

A wayward feature/bug in Hugo 0.63.1 results in the homepage being badly
formatted:

  https://github.com/gohugoio/hugo/issues/6805



This is rectified in 0.63.2, but the docker image for this does not exist
yet.

It seems that it is risky for us to depend on `latest` in the CI runner
as this can result in unpredictable issues with the website. As the
`stable` Hugo branch tracks the latest release, it wouldn't have helped us
in this instance (the issue was present in the latest release) and this
doesn't seemed to be tracked by the docker image registry.

Lock to version 0.62.2, we will have to update this more manually on a
regular basis.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>