Skip Docker Hub login for forked PRs (#2629)
Forked PRs cannot access GitHub secrets (DOCKERHUB_USERNAME and
DOCKERHUB_TOKEN), causing the Docker login step to fail with 'Username
and password required'. This blocks external contributions.
This change makes the Docker Hub login conditional:
- Skips login for forked PRs (secrets unavailable)
- Runs login for PRs from branches in the main repo
- Runs login for push events, tags, and manual workflows
Docker Hub authentication helps avoid rate limits (429 errors) when
pulling base images like python:* and nvidia/cuda:* during builds.
Forked PRs will use anonymous Docker Hub pulls and may occasionally
hit rate limits, but this is an acceptable trade-off to unblock public
contributions.
Fixes the issue reported in PR #2619 where forked PRs fail at the
Docker login step in CI.
Co-authored-by: Mark Phelps <mphelps@cloudflare.com>
