[8.19](backport #5910) ci: explicit dockerhub login

[mergify]

What is the problem this PR solves?

Explicit docker login to those steps that require access to dockerhub

How does this PR solve the problem?

Otherwise quota limit

Being explicit helps to know those third-party dependencies and document what steps use it

How to test this PR locally

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Related issues

#5767 solved the packaging after seeing some quota limits when accessing docker without authentication.

#5392 introduced the steps when testing the release that requires access to dockerhub

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This is an automatic backport of pull request #5910 done by Mergify.