kie-issues#1255: agent docker config in main pipeline

[jstastny-cz]

To properly fix

• githubscm PR-related methods need gh CLI incubator-kie-issues#1255

Adding jenkins agent docker configuration to .ci/config/main.yaml

Also in:

• kie-issues#1255: agent docker config in main pipeline incubator-kie-kogito-pipelines#1205
• kie-issues#1255: agent docker config in main pipeline incubator-kie-drools#5971
• kie-issues#1255: agent docker config in main pipeline #3094

There are jobs already in main pipeline (either root folder or defined in dsl/seed_job_branch.groovy file), which didn't have access to jenkins agent docker image and args configuration from branch.yaml - because that one is loaded only in Jenkinsfile.seed.branch - which did not make it possible to reference these values already in dsl/seed_job_branch.groovy.

So adding the agent docker configuration into main.yaml and loading already in Jenkinsfile.seed.main, so that all DSL jobs are able to use this value loaded from particular branch (except from the dsl/seed_job_main.groovy which is the actual first entry point into DSL generation and no docker config is needed there as of now).