fix(deps): pin dependencies

This MR contains the following updates:

Package	Type	Update	Change
docker	service-image	pinDigest	→ 173f284
docker	image	pinDigest	→ 625d943
python	image	pinDigest	→ 6d58c1a
registry.gitlab.developers.cam.ac.uk/uis/devops/continuous-delivery/gitlab-continuous-delivery-utils/master	image	pinDigest	→ 99dbd35

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