update release 2.6.0 with CDAP docker build info for developers

Perhaps then:

```
* Download the CDAP (GitHub) repo:

$ git clone git@github.com:caskdata/cdap.git

* Checkout the appropriate branch, substituting for <version>:

$ cd cdap; git checkout release/<version>

* Build the Docker image, using the current directory for the context:

$ sudo docker build caskdata/cdap-standalone .

````
Note the addition of ``$`` at the start of the commands.
Note that all the examples at ``docker.com`` include ``sudo`` when running the commands. Is that required? If so, we should include it.
Is the path ``caskdata/cdap-standalone`` correct?

I just tried these commands, and they didn't work. I installed docker -- ran its HelloWorld -- and then download the repo, switched to the release branch, ran the docker command, and got a usage statement. I tried changing it to just the local dir, and got this:
```
$ sudo docker build .
Sending build context to Docker daemon 
FATA[0001] Post http:///var/run/docker.sock/v1.16/build?rm=1&t=: dial unix /var/run/docker.sock: no such file or directory 
```