Dockerfile.git-gate builds a small alpine image with git,
openssh-client, and gitleaks; the directory layout the entrypoint
and per-upstream cp's expect is pre-created in the image so docker
cp can target paths beneath /etc/git-gate and /git-gate/creds at
container-create time (cp doesn't create intermediate dirs).
DockerGitGate.start mirrors DockerSSHGate's shape: build, create,
cp the rendered entrypoint + hook + per-upstream identity files
(plus a known_hosts file synthesized from KnownHostKey when set),
attach the egress network, start. build_image gains an optional
dockerfile= argument so the gate can build from its own
Dockerfile in the shared context.
PRD: docs/prds/0008-git-gate.md
