Jenkins: Add centos6 build job to centos7_docker_image_updater jenkins job

[Haroon-Khel]

• [x] commit message has one of the standard prefixes
• [ ] faq.md updated if appropriate
• [ ] other documentation is changed or added (if applicable)
• [ ] playbook changes run through VPC or QPC (if you have access)
• [ ] VPC/QPC not applicable for this PR
• [ ] for inventory.yml changes, bastillion/nagios/jenkins updated accordingly

ref https://github.com/adoptium/infrastructure/issues/2753

If we're using this job to push the centos6 image to dockerhub, then am I correct to assume that we then do not have to push the image to dockerhub during the github workflow https://github.com/adoptium/infrastructure/blob/de2d533f614be45ea37d34082efc306f1e2f4e74/.github/workflows/build.yml#L20, and just use the pr workflow as a test?

[gdams]

@Haroon-Khel is there a reason to make this change? GitHub actions is a more stable environment and works well for pushing the Centos 6 job where we only need intel machines. I used Jenkins because we had to build arm64 etc for Centos 7

[Haroon-Khel]

@gdams It was really out of a desire to have the these build jobs in one place. In terms of stability, im trying to get the jenkins job to become more stable, https://github.com/adoptium/infrastructure/issues/2760, so after i've got it running green(er) it might make more sense to have the centos6 build job running there than in the workflow