Allow pushing to GH from PVC (or document how to/make easier)

[DominicOram]

Currently, it is not easy to push changes from my scratch PVC back to GH. There are a few reasons for this:

1. The scratch area is pulled using https. To push with https you need to set up a PAT, which people tend not to do and instead use ssh. Even if we did use a PAT here, I'm not sure how to do it securely given that we're using a shared PVC?
2. If you try and pull with ssh you can't as the scratch initialisation doesn't have ssh
3. If you change the remote in the PVC after the fact e.g. git remote set-url origin git@github.com:DiamondLightSource/dodal.git. You then get:

error: cannot run ssh: No such file or directory
fatal: unable to fork

(restarting the pod, as suggested in https://diamondlightsource.slack.com/archives/C051NDJH6BZ/p1787577508010119 does not fix this)

Acceptance Criteria

• It is easy, or at least it is documented, to push code from a PVC