Build both agent and inbound-agent container images in the same repository

[lemeurherve]

I propose to build both images in the https://github.com/jenkinsci/docker-agent/ repository, for faster releases and easier maintenance.

(Opening an issue here for your information/visibility)

See jenkinsci/docker-agents#569 for more details.

Corresponding pull request, ready for review: jenkinsci/docker-agents#570

[lemeurherve]

As jenkinsci/docker-agents#570 has been merged and released with success for both images, proceeding to jenkinsci/docker-agents#569 (comment)