Add a --add-host host:ip flag which appends lines to /etc/hosts.

[thockin]

As per #2335 and #2267 and maybe others, this adds a simple flag to append lines to /etc/hosts.

This is my first foray into go and Docker code, so I'm happy to be told where I screwed it up. I mostly followed the example of the Env flag.

[crosbymichael]

@thockin What is the usecase for doing this?

[thockin]

Specifically I was playing with things Docker can do, and I wanted to be able to identify the host or another container by IP without running DNS. I also wanted to give a name to the bridge endpoint so I could map ports to it without going off-host, mostly to see what I could do with it.

I scanned open issues, and this was a topic of several of them. I wanted an excuse to dig into the code a bit, so I thought I would use this as an excuse to figure things out.

[proppy]

I would add the error handling first.

arr := strings.Split(val, """)
if len(arr) != 2 {
   return "", fmt.Errorf(...)
}
return val, nil

[crosbymichael]

I think we are going to look into #5129 to solve this issue.