Disable ipv6 in docker

Hi @amercader ,

i struggle since 2 days with the message
```ckan-1 | socket(): Address family not supported by protocol [core/socket.c line 82]```
which immediatly crashes my container.

I rebuilt the image just 2 days ago and now the same image tag does not work anymore.
My Dockerfile uses `ckan/ckan-base:2.10.4`.
Why did the Docker Image change?
It seems the container image versioning does not follow semver of ckan.
What is the best practice to guarantee nothing is breaking when using the ckan container?

On my local machine everything runs fine.
The production machine i use, the hoster has disabled ipv6 due to compliance bingo.
I saw you merged https://github.com/ckan/ckan-docker-base/pull/67 two weeks ago, binding to the local ipv6 device by default.
My assumption is this is causing my issue.

In general i'd favor to support ipv6 as well, but ipv6 is often not an option.
Especially in security enhanced networks it is usual to disable ipv6 by default to reduce complexity and attack vectors.

To solve this i propose an environment variable to define the network interface to bind to, binding to ipv6 localhost `[::]` by default, but allowing to override it.

What do you think?