Re-enable new project-scope Dockerhub token

[jade-guiton-dd]

During the latest release (v0.129.0), the CI failed to authenticate to Dockerhub. This was caused by #982, which introduced a new project-scoped Dockerhub token (see open-telemetry/community#2805) instead of the old org-wide token. The PR was reverted, and authentication succeeded. We need to figure out why the authentication failed, and how to fix it, since the org-wide token is slowly being phased out (cf. the issue on the community repo).

[codeboten]

@dmathieu as the contributor on the original PR to update the docker credentials, how can we move forward with this change? Are these credentials stored in a 1password vault that's accessible to other collector maintainers?

[dmathieu]

I have no idea. I made the original PR to provide consistency, as this was the token I was told to use for ebpf-profiler.

@trask owns that token.

[mowies]

i think it would be good to create a test repo to try out if the token actually works. we have no way of properly testing the release otherwise currently

[mowies]

i added a quick regression test workflow that should hopefully catch docker token errors in the future: #1001

[jade-guiton-dd]

From #1004, looks like we found the culprit: we also needed to change secrets.DOCKER_USERNAME to vars.DOCKER_USERNAME. So I guess we just need to re-file #982 with that change.