…e template

The custom-base-image eval template (Dockerfile.custom.template) never
copied the docker-agent binary nor set the /run.sh entrypoint wrapper, so
eval containers inherited the base image's ENTRYPOINT ["/docker-agent"].
The agent YAML path was then passed as a bare subcommand, failing every
custom-image eval with:

    Error: unknown command "/configs/<agent>.yaml" for "docker-agent"

Bring the custom template to parity with the default template by copying
the binary from docker/docker-agent:edge and wrapping it with the
/run.sh "docker-agent run --exec --yolo --json" entrypoint. Add a
regression test asserting parity between the two templates.

Fixes #796