Build in custom docker image

Please consider `nginx` crate docs: https://docs.rs/nginx/0.4.0/nginx/

as you can see it is not built because the crate needs some modification in the OS:

```sh
sed -i 's:# define IPPORT_RESERVED:// #define IPPORT_RESERVED:' /usr/include/netdb.h
```

Is it possible to provide a custom docker image to build docs for such crates?
