Build nightly image on every PR

[lrlna]

To make it easier to verify changes introduced in a given PR, this work introduces building nightly images for every PR.

Since we'll be creating a few more images than before, I am also adding a workflow for deleting old nightlies (nightly/router docker assets)

For the time being, I chose to keep with the nightly package rather than creating a new one just for PRs. Let me know if you are of a different opinion.

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Checklist

Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• [x] Changes are compatible[^1]
• [x] Documentation[^2] completed
• [ ] Performance impact assessed and acceptable
• Tests added and passing[^3]
  • [ ] Unit Tests
  • [ ] Integration Tests
  • [ ] Manual Tests

[^1]: It may be appropriate to bring upcoming changes to the attention of other (impacted) groups. Please endeavour to do this before seeking PR approval. The mechanism for doing this will vary considerably, so use your judgement as to how and when to do this. [^2]: Configuration is an important part of many changes. Where applicable please try to document configuration examples. [^3]: Tick whichever testing boxes are applicable. If you are adding Manual Tests, please document the manual testing (extensively) in the Exceptions.