Add support for building Agent containers to the CI

[webbnh]

This PR adds support to the CI for building a single set of Agent containers suitable for running functional tests.

The CI pipeline builds a full set of Agent containers (base, tools, tool-meister, tool-data-sink, workloads, and all) for a single distro (currently Fedora-36) and pushes them to the container registry. For PR builds, it pushes them to the internal registry; for branch builds (like main), it pushes them to the external registry.

Details:

• Modify the Agent containers Makefile:
  • allow the image repository and the image tag to be specified by command line arguments
  • add a special target for use by the CI for pushing images to the repository: this target pushes the target distribution's container-set to the specified repository using the specified tag, without adding further complexity to the existing push script (i.e., without adding another entry in the TAG_TYPES list)
  • explicitly declare as "phony" all the phony targets
• Modify the Jenkins Pipeline:
  • add support for referencing both internal and external container image registries
  • make use of per-stage environment directives where appropriate instead of defining variables for the entire pipeline
    rename the local references of PB_SERVER_IMAGE_TAG to PB_IMAGE_TAG so it can be used for both the Server and Agent container images; define PB_SERVER_IMAGE_TAG to the value of PB_IMAGE_TAG in the stages which need it; use a better idiom to generate its value
  • add a stage which builds the Agent container images and pushes them to the registry

PBENCH-332

[dbutenhof]

You asked for review, but it's Draft: so a few comments but no cookie.

[webbnh]

Ready for review, at last.

[portante]

I'd like to know why we have to use the internal registry for container images build for PRs when there is not publishing restriction.