Pin container images

[JimMadge]

✅ Checklist

• You have given your pull request a meaningful title (e.g. Enable foobar integration rather than 515 foobar).
• You are targeting the appropriate branch. If you're not certain which one this is, it should be develop.
• Your branch is up-to-date with the target branch (it probably was when you started, but it may have changed since then).
• You have marked this pull request as a draft and added '[WIP]' to the title if needed (if you're not yet ready to merge).
• You have formatted your code using appropriate automated tools (for example ./tests/AutoFormat_Powershell.ps1 -TargetPath <path to file or directory> for Powershell).

⤴️ Summary

Pin unpinned container image version. In particular nexus-allowlist which was using the 'main' tag that could point to unstable/broken builds.

This is also consistent with other container instance specifications in the project.

I think there is a discussion around what level to pin to. I have tried to pin to major versions where possible as this a avoids known, breaking changes being introduced.
Pinning to minor or patch versions gives more confidence that there will be no regressions, however it adds more burden to update versions more regularly.

🌂 Related issues

🔬 Tests

[jemrobinson]

LGTM