Remove the container deployment

Remove the container deployment

A Vole instance is one process whose state is a directory on local disk. A
container therefore adds a layer without removing one, and it costs
something real: the service unit's hardening, and the CI half, which needs
an OCI runtime the container does not have without mounting the host's
socket — the thing the runner docs refuse.

It was also the weakest path in practice. No image is published, so an
upgrade meant a source checkout and a Go build where the package is one
`apt install`. CI never ran the container; it was tested by hand once, which
is a surface that rots quietly.

So the Dockerfile, the compose file, the entrypoint, the image's
configuration, and the guide go. The deployment checker loses its compose
and COPY checks, and the render script is the only shipped script left.

Every other mention of Docker in the repository means the CI container
runtime, and those stay.

Users who manage their machines with Compose or Kubernetes are told where
they stand: give the forge a small virtual machine of its own, and the
first-boot file works there unchanged.