Login to docker hub only if credentials were provided

[yarikoptic]

I think this could be the only solution (may be besides also adding retries in addition) to allow testing against dandi-cli to be done also in PRs from forks of the project, where there is no credentials. We could also advise our developers to provide such (own) credentials in their forks.

FTR: login originally was added in 6b815dc

• I think we can state that it closes CI can't read GitHub Actions secrets on fork-based PRs #2107 . If it fails to pull from docker -- it would be a differently titled issue ;-)
• note: this PR submitted from a personal fork, so would test ability to pass for similar ones

[yarikoptic]

success as we did build image without credentials:

Not login-in to docker since no credentials were provided, might hit limits below
14
#0 building with "default" instance using docker driver
15

16
#1 [internal] load build definition from django-public.Dockerfile
17
#1 transferring dockerfile: 373B done
18
#1 DONE 0.0s

the fails are actually due to that

• Capitalize Archive when it follows DANDI #2114 was not "complete" and thus broke it and started to fail in master, e.g. https://github.com/dandi/dandi-archive/actions/runs/12415512591

I will be addressing @jwodder comment and will see if could fix it while at this

[yarikoptic]

scrape above "comment" - sure thing it broke testing of dandi-cli since we need now to fix it up there too! ;-)

[yarikoptic]

FTR -- build image is fine again!

[dandibot]

🚀 PR was released in v0.4.2 🚀