buildkite: docker login

[v1v]

this should help with environmental issues when accessing docker.hub

Motivation/summary

Docker login so no issues with quota limit in Dockerhub.

Fixes

ERROR: failed to solve: golang:1.20.7: failed to copy: httpReadSeeker: failed open: unexpected status code 
https://registry-1.docker.io/v2/library/golang/manifests/sha256:7fcc2684ef0de5f7730be51a493d26289756356a3c385ce6374b86d31d0be446: 429 Too Many Requests - Server message: toomanyrequests: 
You have reached your pull rate limit. You may increase the limit by authenticating and upgrading: https://www.docker.com/increase-rate-limit
--
  | make: *** [packaging.mk:42: build/docker/apm-server-8.10.0-SNAPSHOT.txt] Error 1

Checklist

• Update CHANGELOG.asciidoc
• Update package changelog.yml (only if changes to apmpackage have been made)
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

In the CI, this is a feature branch so it triggered this build in Buildkite and docker login was done as part of the pre-command hook

Related issues