A problem affecting the build of the Mosquitto container keeps showing
up in Discord questions. Examples:

* [2021-09-17](https://discord.com/channels/638610460567928832/638610461109256194/888096248761045022)
* [2021-09-09](https://discord.com/channels/638610460567928832/638610461109256194/885494986710335498)

The problem is discussed in [alpinelinux/docker-alpine issues/98](alpinelinux/docker-alpine#98).

It is not clear whether:

1. The problem is transient (ie those reporting it are able to get past
the problem on a retry);
2. Only affects Mosquitto or potentially affects other Alpine-based
IOTstack containers using `apk` to add packages (eg Node-RED); or
3. Environmental (eg if there is a proxy system between the Raspberry Pi
and dl-cdn.alpinelinux.org).

This Pull Request is implementing the patch suggested by Issue 98 of
reverting `apk` requests to use HTTP.

Given the march towards HTTPS-everywhere, reverting to HTTP might seem
inadvisable but:

* Issue 98 was opened in July 2020.
* There seems to have been no significant progress towards its
resolution since January 2021.
* The Discord traffic suggests it is an ongoing and present issue for
IOTstack users.

Adds health-check script to template.

Alters Dockerfile to import health-check script and adds `HEALTHCHECK`
directive.

Updates container documentation to explain behaviour of, and
configuration options for, the health-check agent.