Update Custom Docker Build Push workflow to use actions/checkout@v6

[Exairnous]

The current version of actions/checkout that is used in the workflow files is v4, but the latest version is v6. If possible, it's good maintenance to keep these utilities up to date, so it would be good to update it.

Possible benefits:

• More secure credential handling (although I don't believe this is currently used in the Custom Docker Build Push workflow).
• Better support for handling tags (although I don't believe this is currently used in the Custom Docker Build Push workflow).

Possible issues:

• The Node version it uses has increased from 20 to 24.
• The minimum compatible GitHub Runner version is v2.327.1

Repositories to update:

• certbotbot (workflow/repository not present yet)
  • PR:
• coturn (workflow/repository not present yet)
  • PR:
• dialog (workflow not present yet)
  • PR:
• hubs
  • PR:
• nearspark
  • PR:
• pgbouncer (workflow/repository not present yet)
  • PR:
• photomnemonic
  • PR:
• reticulum
  • PR:
• speelycaptor (workflow not present yet)
  • PR:
• spoke
  • PR:

[DougReeder]

For nearspark, using checkout@v6 appears to work fine.
https://github.com/DougReeder/nearspark/actions/runs/21886204929/job/63181590807

I tested the resulting image with the following commands, and the image appears to work fine:
kubectl set image deployment/nearspark nearspark=dougreeder/nearspark:master-3 -n hcce
kubectl rollout status deploy nearspark -n hcce
kubectl logs -l app=nearspark -f -n hcce

[DougReeder]

For photomnemonic, using checkout@v6 appears to work fine:
https://github.com/DougReeder/photomnemonic/actions/runs/21893036067/job/63203023683

I tested the resulting image with the following commands, and the image appears to work fine:
kubectl set image deployment/photomnemonic photomnemonic=dougreeder/photomnemonic:master-2 -n hcce
kubectl rollout status deploy photomnemonic -n hcce
kubectl logs -l app=photomnemonic -f -n hcce