Add method to get container IP address

[robdimsdale]

Summary

Add a helper method to a running Container object to obtain the IP address of the container on a given network.

Use Cases

This makes it easy to get the IP address of a running container for a given network name. Without this, you essentially have to run docker inspect and manually traverse the returned json.

Given that the default network is so often called bridge, I could see an argument for adding a second helper method to obtain the IP address on this specific network - something like this:

func (c Container) IPAddressForBridgeNetwork() string)

Signed-off-by: Sophie Wigmore [email protected]

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).