Docker update action currently broken

:white_check_mark: Checklist

[x] I have searched open and closed issues for duplicates.
[x] This is a problem observed when deploying a Data Safe Haven.
[x] I can reproduce this with the latest version.
[x] I have read through the documentation.
[x] This isn't an open-ended question (open a discussion if it is).

:no_entry_sign: Describe the problem

See recent runs (e.g. https://github.com/alan-turing-institute/data-safe-haven/actions/runs/9343591806) which have failed

:deciduous_tree: Log messages

Relevant log messages

```shell > python .github/scripts/update_docker_versions.py Traceback (most recent call last): File "/home/runner/work/data-safe-haven/data-safe-haven/.github/scripts/update_docker_versions.py", line 85, in Updating apricot from 0.0.5 to 0.0.6 in data_safe_haven/infrastructure/programs/sre/identity.py v_latest = sorted(stable_versions, key=lambda v: v[1], reverse=True)[0][0] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^ IndexError: list index out of range Updating redis from 7.2.4 to 7.2.5 in data_safe_haven/infrastructure/programs/sre/identity.py Updating adguardhome from v0.107.48 to v0.107.50 in data_safe_haven/infrastructure/programs/sre/dns_server.py Updating caddy from 2.7.6 to 2.8.1 in data_safe_haven/infrastructure/programs/sre/hedgedoc_server.py Leaving hedgedoc at 1.9.9 (latest version) in data_safe_haven/infrastructure/programs/sre/hedgedoc_server.py Updating caddy from 2.7.6 to 2.8.1 in data_safe_haven/infrastructure/programs/sre/software_repositories.py Updating nexus3 from 3.67.1 to 3.68.1 in data_safe_haven/infrastructure/programs/sre/software_repositories.py Leaving nexus-allowlist at v0.9.0 (latest version) in data_safe_haven/infrastructure/programs/sre/software_repositories.py ```

:recycle: To reproduce

Trigger the Docker update GitHub action

alan-turing-institute / data-safe-haven