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

doc/docker.md

@@ -5,34 +5,34 @@ The easiest/faster option is to use the latest image.
 Let´s first check the version we have. The first time you run this command, the polkadot docker image will be downloaded. This takes a bit of time and bandwidth, be patient:
 
 ```bash
-docker run --rm -it chevdor/polkadot:0.3.14 polkadot --version
+docker run --rm -it chevdor/polkadot:latest polkadot --version
 ```
 
 You can also pass any argument/flag that polkadot supports:
 
 ```bash
-docker run --rm -it chevdor/polkadot:0.3.14 polkadot --chain alex --name "PolkaDocker"
+docker run --rm -it chevdor/polkadot:latest polkadot --chain westend --name "PolkaDocker"
 ```
 
 Once you are done experimenting and picking the best node name :) you can start polkadot as daemon, exposes the polkadot ports and mount a volume that will keep your blockchain data locally:
 
 ```bash
-docker run -d -p 30333:30333 -p 9933:9933 -v /my/local/folder:/data chevdor/polkadot:0.3.14 polkadot --chain alex
+docker run -d -p 30333:30333 -p 9933:9933 -v /my/local/folder:/data chevdor/polkadot:latest polkadot --chain westend
 ```
 
 Additionally if you want to have custom node name you can add the `--name "YourName"` at the end
 
 ```bash
-docker run -d -p 30333:30333 -p 9933:9933 -v /my/local/folder:/data chevdor/polkadot:0.3.14 polkadot --chain alex --name "PolkaDocker"
+docker run -d -p 30333:30333 -p 9933:9933 -v /my/local/folder:/data chevdor/polkadot:latest polkadot --chain westend --name "PolkaDocker"
 ```
 
 ```bash
-docker run -d -p 30333:30333 -p 9933:9933 -v /my/local/folder:/data chevdor/polkadot:0.3.14 polkadot --rpc-external --chain alex
+docker run -d -p 30333:30333 -p 9933:9933 -v /my/local/folder:/data chevdor/polkadot:latest polkadot --rpc-external --chain westend
 ```
 
 if you want to connect to rpc port 9933, then must add polkadot startup parameter: `--rpc-external`.
 
-**Note:** The `--chain alex` argument is important and you need to add it to the command line. If you are running older node versions (pre 0.3) you don't need it.
+**Note:** The `--chain westend` argument is important and you need to add it to the command line. If you are running older node versions (pre 0.3) you don't need it.
 
 ### Limiting Resources
 
@@ -68,7 +68,7 @@ If you run into issues with polkadot when using docker, please run the following
 (replace the tag with the appropriate one if you do not use latest):
 
 ```bash
-docker run --rm -it chevdor/polkadot:latest polkadot version
+docker run --rm -it chevdor/polkadot:latest polkadot --version
 ```
 
 This will show you the polkadot version as well as the git commit ref that was used to build your container.