runner: mount the build home into the container !311

Found by the smoke build: HOME inside the container is /var/lib/gitbay-runner/work/home, the persistent cache directory, but nothing mounted it. Every containerised build would start with a cold Go module cache and re-download the sonar scanner — the regression the build home was added to fix, back again under podman.

The build home is now bind-mounted at the same path, so HOME resolves identically with and without a container. Only that directory: the workdir above it holds other builds' workspaces and must not be visible.

Stacked on !310.

Ref #144