feat: enable image build spec for tasks

[tedim52]

Description

ex.

def run(plan, args):
    result = plan.run_sh(
        run="echo ${SOME}",
        # build image containing openssl, gh cli, python, and bash, etc.
        image=ImageBuildSpec(
            build_context_dir="./",
            image_name="taskimg",
        ),
        env_vars={
            "SOME": "THING",
        }
    )
    plan.print(result)

Is this change user facing?

YES

References:

#2224