ci(stage2): login to Docker Hub

[janishorsts]

Fixes https://github.com/EarthBuild/earthbuild/actions/runs/18553354024/job/52885583589

Error: failed to SAVE IMAGE: failed to authorize: failed to fetch oauth token: unexpected status from GET request to https://auth.docker.io/token?scope=repository%3Aearthbuild%2Fexamples%3Apull%2Cpush&service=registry.docker.io: 401 Unauthorized
Error: failed to SAVE IMAGE: failed to authorize: failed to fetch oauth token: unexpected status from GET request to https://auth.docker.io/token?scope=repository%!A(MISSING)earthbuild%!F(MISSING)examples%!A(MISSING)pull%!C(MISSING)push&service=registry.docker.io: 401 Unauthorized
Error: Process completed with exit code 1.

[github-actions]

➖ Are we earthbuild yet?

No change in "earthly" occurrences

📈 Overall Progress

Branch	Total Count
main	6068
This PR	6068
Difference	+0

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Keep up the great work migrating from Earthly to Earthbuild! 🚀

💡 Tips for finding more occurrences

Run locally to see detailed breakdown:

./.github/scripts/count-earthly.sh

Note that the goal is not to reach 0.
There is anticipated to be at least some occurences of earthly in the source code due to backwards compatibility with config files and language constructs.