Update Docker Quickstart to use 'runsc install'

[ianlewis]

https://gvisor.dev/docs/user_guide/quick_start/docker/#configuring-docker

We should tell folks to use the 'runsc install' command to configure docker since that's easier. We also don't need to configure Docker if they installed via 'apt' so we should either move these instructions to the installation page, or make it clear that they don't have to do this if they used 'apt'.

[ianlewis]

We should probably update places that use 'runtimeArgs' to use 'runsc install' too. ex:

Here we could do sudo runsc install -- --network=host to enable host networking for example.