Add Dockerfile.deploy as a thin wrapper that pulls
ghcr.io/fabro-sh/fabro:nightly, and point railway.toml at it. Railway
now skips Rust compilation entirely and deploys in seconds. The
upstream image already configures entrypoint, $PORT-aware CMD, volumes,
and the unprivileged fabro user, so nothing else is needed in the
wrapper.
Update docs/administration/deploy-railway.mdx to reflect the new flow
and call out that amd64 is the supported architecture (arm64 variant
of the image is being handled separately).
Verified locally: `docker build -f Dockerfile.deploy .` succeeds,
`fabro version` prints the expected banner, and PORT override + health
check work.
