Ensure agent profile is executed in agent container setup

[portante]

If we wanted to...we could set /opt/pbench-agent/profile as the entrypoint for the container, so that it is run automagically when the container is invoked. If we then tweaked that script to, as its last action, run ${@}, any command line provided to the container invocation would then be run in that environment.

That would remove the need for this step, and it would happen seamlessly and automatically for all uses of the container (unless someone overrode the entrypoint), which seems like a really useful idea, given the purpose of this container.

Thoughts? (If we didn't want to disturb /opt/pbench-agent/profile itself, we could wrap it in another script (a la the jenkins/runner) and point the entrypoint at that.)

Originally posted by @webbnh in #3259 (comment)