Add a dedicated page documenting how to run the OpenShell gateway as a
container using docker run, docker-compose, or podman, without the
system package manager installer.
This is useful for users on immutable OS distributions (Fedora CoreOS,
bootc-based images, Silverblue) where the standard install.sh path is
not appropriate, and for container-first environments.
Covers a quick-start with TLS disabled (localhost-bound), a full mTLS
setup using the gateway's generate-certs subcommand, a docker-compose
example, and a Podman variant.
Closes the gap raised in #1285.
Signed-off-by: Eric Curtin <eric.curtin@docker.com>
