feat: add timeout when waiting for docker in .docker-in-docker fragment

Closes #183 (closed)

This MR improves the before_script of the .docker-in-docker fragment, adding a timeout and bail out after 30 seconds if docker does not come up.

This takes the code originally written by @hwtb2 in https://gitlab.developers.cam.ac.uk/uis/devops/digital-admissions/pools/smi/-/merge_requests/383, and adjusts it slightly to incorporate the comment in the issue #183 (comment 1725576).

I am happy with any suggestions. I feel like this is already an improvement over what we have. If we feel like we can do even better, then I would suggest still merging this, and keeping #183 (closed) open for further improvements.