Use --offline when building docs

[jsha]

This reflects the intent of the command, since containers are isolated from the network. And it provides a clearer error message in case there is some problem and cargo would try to make a network request.

When building with (e.g.) CARGO_UNSTABLE_SPARSE_REGISTRY=true on the host, this produces errors like this:

2022/10/15 01:57:02 [INFO] rustwide::cmd: [stderr] error: failed to download `autocfg v1.1.0`
2022/10/15 01:57:02 [INFO] rustwide::cmd: [stderr]
2022/10/15 01:57:02 [INFO] rustwide::cmd: [stderr] Caused by:
2022/10/15 01:57:02 [INFO] rustwide::cmd: [stderr]   attempting to make an HTTP request, but --offline was specified

Related: #1881