Update docker.md (#802)

* Update docker.md

Few Edits:
 - Use the `latest` version of polkadot.
 - Add a missing `--` when reporting the version.

* Update docker.md use `westend` chain

> Alex doesn't exist anymore, the current test net is `westend`.
According to https://github.com/paritytech/polkadot/issues/803
8 jobs for kusama-nightly-staging in 20 minutes and 14 seconds (queued for 1 second)