The `halfshot/matrix-hookshot` container images published to Docker Hub
(as of 2022-04-05, at least) are only available for `amd64`, not for
`arm64`. Self-building on arm64 is necessary.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1728

It should be noted that a `roiarthurb/matrix-hookshot` container image is available,
which is available for the arm64 platform, but that's non-official and doesn't
contain an amd64 build, so it's of limited use.